Replace buffer I/O locks with condition variables.

1.  Backends waiting for buffer I/O are now interruptible.

2.  If something goes wrong in a backend that is currently performing
I/O, waiting backends no longer wake up until that backend reaches
AbortBufferIO() and broadcasts on the CV.  Previously, any waiters would
wake up (because the I/O lock was automatically released) and then
busy-loop until AbortBufferIO() cleared BM_IO_IN_PROGRESS.

3.  LWLockMinimallyPadded is removed, as it would now be unused.

Author: Robert Haas <robertmhaas@gmail.com>
Reviewed-by: Thomas Munro <thomas.munro@gmail.com>
Reviewed-by: Julien Rouhaud <rjuju123@gmail.com>
Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us> (earlier version, 2016)
Discussion: https://postgr.es/m/CA%2BhUKGJ8nBFrjLuCTuqKN0pd2PQOwj9b_jnsiGFFMDvUxahj_A%40mail.gmail.com
Discussion: https://postgr.es/m/CA+Tgmoaj2aPti0yho7FeEf2qt-JgQPRWb0gci_o1Hfr=C56Xng@mail.gmail.com

doc/src/sgml/monitoring.sgml

@@ -1586,6 +1586,10 @@ postgres   27093  0.0  0.0  30096  2752 ?        Ss   11:34   0:00 postgres: ser
       <entry>Waiting for the page number needed to continue a parallel B-tree
        scan to become available.</entry>
      </row>
+     <row>
+      <entry><literal>BufferIO</literal></entry>
+      <entry>Waiting for buffer I/O to complete.</entry>
+     </row>
      <row>
       <entry><literal>CheckpointDone</literal></entry>
       <entry>Waiting for a checkpoint to complete.</entry>
@@ -1876,10 +1880,6 @@ postgres   27093  0.0  0.0  30096  2752 ?        Ss   11:34   0:00 postgres: ser
       <entry><literal>BufferContent</literal></entry>
       <entry>Waiting to access a data page in memory.</entry>
      </row>
-     <row>
-      <entry><literal>BufferIO</literal></entry>
-      <entry>Waiting for I/O on a data page.</entry>
-     </row>
      <row>
       <entry><literal>BufferMapping</literal></entry>
       <entry>Waiting to associate a data block with a buffer in the buffer