Remove the BtreeMutexArray object - use the Vdbe.btreeMask field to accomplish

the same result.  Add a generation counter to btree mutexes in order to assert
that mutexes are never temporarily dropped over a range of instructions in order
to do deadlock avoidance in some subroutine.  Lock all btrees in any Vdbe
program that uses OP_ParseSchema.

FossilOrigin-Name: d81708f7d1eee399bfe76f6b8dac950a85dc2582

src/btreeInt.h

@@ -426,7 +426,7 @@ struct BtShared {
   u32 nPage;            /* Number of pages in the database */
   void *pSchema;        /* Pointer to space allocated by sqlite3BtreeSchema() */
   void (*xFreeSchema)(void*);  /* Destructor for BtShared.pSchema */
-  sqlite3_mutex *mutex; /* Non-recursive mutex required to access this struct */
+  sqlite3_mutex *mutex; /* Non-recursive mutex required to access this object */
   Bitvec *pHasContent;  /* Set of pages moved to free-list this transaction */
 #ifndef SQLITE_OMIT_SHARED_CACHE
   int nRef;             /* Number of references to this structure */
@@ -435,6 +435,7 @@ struct BtShared {
   Btree *pWriter;       /* Btree with currently open write transaction */
   u8 isExclusive;       /* True if pWriter has an EXCLUSIVE lock on the db */
   u8 isPending;         /* If waiting for read-locks to clear */
+  u16 iMutexCounter;    /* The number of mutex_leave(mutex) calls */
 #endif
   u8 *pTmpSpace;        /* BtShared.pageSize bytes of space for tmp use */
 };