Checkpoint.

- Fix a couple of edge cases while switching between collections.
  - Raise error if attempting to switch collections mid-transaction.
  - Add logic for re-entrancy with long-running transactions.
  - Revert to the prior collection if passing a block to #collection.
  - Add a predicate to tell if you're currently within a long-running transaction.
  - Add separate commit/rollback to long-running transactions.

FossilOrigin-Name: 711239e6fc2f25479a26fb54805da1e5db792f97f28a8b5724e0b38eb11cdb07

ext/mdbx_ext/mdbx_ext.h

@@ -31,6 +31,7 @@ struct rmdbx_db {
 
     struct {
        int open;
+       int retain_txn;
     } state;
 
 	char *path;