Avoid unnecessary calls to sqlite3BtreeEnterAll() and sqlite3BtreeLeaveAll()

when no btree is using shared-cache.

FossilOrigin-Name: cfb3158204628eb2fd170090a7f212df0e4ce6c9

src/sqliteInt.h

@@ -1296,6 +1296,7 @@ struct sqlite3 {
   u8 vtabOnConflict;            /* Value to return for s3_vtab_on_conflict() */
   u8 isTransactionSavepoint;    /* True if the outermost savepoint is a TS */
   u8 mTrace;                    /* zero or more SQLITE_TRACE flags */
+  u8 skipBtreeMutex;            /* True if no shared-cache backends */
   int nextPagesize;             /* Pagesize after VACUUM if >0 */
   u32 magic;                    /* Magic number for detect library misuse */
   int nChange;                  /* Value returned by sqlite3_changes() */