Implement "drop_sessions()" method

See merge request !2

Coding Standards & Unit Tests

See merge request !1

ecac6b63 changed the constant names so they could be set independently of the Redis Object Cache plugin's options, but the readme wasn't updated accordingly.

Give plugin its own Redis config namespace. Since we're using defines, user can chain defines as needed.

* Port Redis PECL library connection from [Redis Object Cache](https://github.com/ericmann/Redis-Object-Cache/commit/b768b03f9dce21edd91806dc1ca741aa9523db7e)
* Replace meta functions with appropriate Redis calls
* Override default session manager

From https://core.trac.wordpress.org/browser/trunk/src/wp-includes/session.php?rev=29635