Merge 'core: Enforce shared database object per database file' from Pekka Enberg

We need to ensures that there is a single, shared `Database` object per
a database file. We need because it is not safe to have multiple
independent WAL files open because coordination happens at process-level
POSIX file advisory locks.
Fixes #2267

Reviewed-by: Pere Diaz Bou <pere-altea@homail.com>

Closes #2299

sqlite3/tests/compat/mod.rs

@@ -355,6 +355,7 @@ mod tests {
                     ),
                     SQLITE_OK
                 );
+                assert_eq!(sqlite3_close(db), SQLITE_OK);
             }
             let mut wal_path = temp_file.path().to_path_buf();
             assert!(wal_path.set_extension("db-wal"));
@@ -380,6 +381,7 @@ mod tests {
                         assert_eq!(sqlite3_step(stmt), SQLITE_DONE);
                         assert_eq!(sqlite3_finalize(stmt), SQLITE_OK);
                     }
+                    assert_eq!(sqlite3_close(db), SQLITE_OK);
                 }
             }
 
@@ -459,6 +461,7 @@ mod tests {
                     ),
                     SQLITE_OK
                 );
+                assert_eq!(sqlite3_close(db), SQLITE_OK);
             }
             let mut wal_path = temp_file.path().to_path_buf();
             assert!(wal_path.set_extension("db-wal"));
@@ -483,6 +486,7 @@ mod tests {
                         assert_eq!(sqlite3_step(stmt), SQLITE_DONE);
                         assert_eq!(sqlite3_finalize(stmt), SQLITE_OK);
                     }
+                    assert_eq!(sqlite3_close(db), SQLITE_OK);
                 }
             }