Trigger events before and after drop table

[vanchaxy]

• fixes The drop table operation does not trigger the before_drop and after_drop events #245

Checklist:

• Add tests that demonstrate the correct behavior of the change. Tests should fail without the change.
• Add or update relevant docs, in the docs folder and in code.
• Ensure PR doesn't contain untouched code reformatting: spaces, etc.
• Run flake8 and fix issues.
• Run pytest no tests failed. See https://clickhouse-driver.readthedocs.io/en/latest/development.html.

[vanchaxy]

Can't manage to work alembic test suite, so only test for table.drop() added. op.drop_table() will emit events when used in migrations after these changes.

[coveralls]

Coverage: 85.376% (-0.05%) from 85.431% when pulling d3eb7b9 on vanchaxy:master into bf3f3e8 on xzkostyan:master.