Problem
=======
Server crashes, when one thread installs and uninstalls plugin of the plugin
type and second thread connects and disconnects.

Analysis
========
Session thread acquires audit plugin at connect. Plugin acquisition is done
using locking/unlocking same mutex multiple times. Server crashes, when
the plugin is uninstalled during plugin acquisition between mutex unlock-lock.

Fix
===
When the plugin is unavailable for acquisition, it's simply skipped.
The plugin is acquired on the next event notification.

RB: 21236
Approved by: Georgi Kodinov <georgi.kodinov@oracle.com>
Approved by: Murthy Sidagam <venkata.sidagam@oracle.com>