Prevent concurrent SimpleLruTruncate() for any given SLRU.

The SimpleLruTruncate() header comment states the new coding rule.  To
achieve this, add locktype "frozenid" and two LWLocks.  This closes a
rare opportunity for data loss, which manifested as "apparent
wraparound" or "could not access status of transaction" errors.  Data
loss is more likely in pg_multixact, due to released branches' thin
margin between multiStopLimit and multiWrapLimit.  If a user's physical
replication primary logged ":  apparent wraparound" messages, the user
should rebuild standbys of that primary regardless of symptoms.  At less
risk is a cluster having emitted "not accepting commands" errors or
"must be vacuumed" warnings at some point.  One can test a cluster for
this data loss by running VACUUM FREEZE in every database.  Back-patch
to 9.5 (all supported versions).

Discussion: https://postgr.es/m/20190218073103.GA1434723@rfd.leadboat.com

doc/src/sgml/catalogs.sgml

@@ -9005,7 +9005,8 @@ SCRAM-SHA-256$<replaceable>&lt;iteration count&gt;</replaceable>:<replaceable>&l
    and general database objects (identified by class OID and object OID,
    in the same way as in <structname>pg_description</structname> or
    <structname>pg_depend</structname>).  Also, the right to extend a
-   relation is represented as a separate lockable object.
+   relation is represented as a separate lockable object, as is the right to
+   update <structname>pg_database</structname>.<structfield>datfrozenxid</structfield>.
    Also, <quote>advisory</quote> locks can be taken on numbers that have
    user-defined meanings.
   </para>
@@ -9031,6 +9032,7 @@ SCRAM-SHA-256$<replaceable>&lt;iteration count&gt;</replaceable>:<replaceable>&l
        Type of the lockable object:
        <literal>relation</literal>,
        <literal>extend</literal>,
+       <literal>frozenid</literal>,
        <literal>page</literal>,
        <literal>tuple</literal>,
        <literal>transactionid</literal>,

doc/src/sgml/monitoring.sgml

@@ -849,7 +849,7 @@ postgres   27093  0.0  0.0  30096  2752 ?        Ss   11:34   0:00 postgres: ser
 
       <tbody>
        <row>
-        <entry morerows="64"><literal>LWLock</literal></entry>
+        <entry morerows="66"><literal>LWLock</literal></entry>
         <entry><literal>ShmemIndexLock</literal></entry>
         <entry>Waiting to find or allocate space in shared memory.</entry>
        </row>
@@ -1047,6 +1047,16 @@ postgres   27093  0.0  0.0  30096  2752 ?        Ss   11:34   0:00 postgres: ser
          <entry>Waiting to execute <function>txid_status</function> or update
          the oldest transaction id available to it.</entry>
         </row>
+        <row>
+         <entry><literal>WrapLimitsVacuumLock</literal></entry>
+         <entry>Waiting to update limits on transaction id and multixact
+         consumption.</entry>
+        </row>
+        <row>
+         <entry><literal>NotifyQueueTailLock</literal></entry>
+         <entry>Waiting to update limit on notification message
+         storage.</entry>
+        </row>
         <row>
          <entry><literal>clog</literal></entry>
          <entry>Waiting for I/O on a clog (transaction status) buffer.</entry>
@@ -1132,7 +1142,7 @@ postgres   27093  0.0  0.0  30096  2752 ?        Ss   11:34   0:00 postgres: ser
          counters during Parallel Hash plan execution.</entry>
         </row>
         <row>
-         <entry morerows="9"><literal>Lock</literal></entry>
+         <entry morerows="10"><literal>Lock</literal></entry>
          <entry><literal>relation</literal></entry>
          <entry>Waiting to acquire a lock on a relation.</entry>
         </row>
@@ -1140,6 +1150,12 @@ postgres   27093  0.0  0.0  30096  2752 ?        Ss   11:34   0:00 postgres: ser
          <entry><literal>extend</literal></entry>
          <entry>Waiting to extend a relation.</entry>
         </row>
+        <row>
+         <entry><literal>frozenid</literal></entry>
+         <entry>Waiting to
+         update <structname>pg_database</structname>.<structfield>datfrozenxid</structfield>
+         and <structname>pg_database</structname>.<structfield>datminmxid</structfield>.</entry>
+        </row>
         <row>
          <entry><literal>page</literal></entry>
          <entry>Waiting to acquire a lock on page of a relation.</entry>