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

.pryrc

@@ -11,5 +11,5 @@ rescue Exception => e
 		e.backtrace.join( "\n\t" )
 end
 
-db = MDBX::Database.open( 'tmp/testdb', max_collections: 50 )
+db = MDBX::Database.open( 'tmp/testdb', max_collections: 5 )