Implement PyMySQL#372: Support session state tracking for GTIDs, sche…

…ma, variables, TX state

Add several methods and two new types for session state tracking:
- `session_state_changed` indicates whether session state has changed
- `get_session_gtids` returns a list of GTIDs created by the just-committed transaction, if applicable
- `get_session_new_schema` returns the new schema, if the schema changed
- `get_session_changed_variables` returns any changed session variables, if any
- `get_session_transaction_state` returns the transaction state, if any and available
- `get_session_transaction_characteristics` returns the transaction characteristics, if any and available

.gitignore

@@ -12,3 +12,4 @@ build/
 dist/
 MySQLdb/release.py
 .coverage
+.idea