Improve audit log handling code

Summary

The audit log events are processed by a thread that can silently fail and stop running.

Steps to Reproduce

Using PostgreSQL to easily introduce a failure writing to the audit table

1. Revoke privileges for the Bamboo database user on the audit table:

   REVOKE ALL PRIVILEGES ON TABLE AUDIT_LOG FROM bamboo;
   

2. Make a change to a plan that will be audited such as overriding concurrent max builds: Plan Configuration >> Miscellaneous
3. There will be an exception writing to AUDIT_LOG table
4. The audit log thread will die and nothing further will log even after restoring permission:

   ALTER TABLE AUDIT_LOG OWNER TO bamboo;
   GRANT ALL PRIVILEGES ON TABLE AUDIT_LOG TO bamboo;
   

Expected Results

Audit log thread is robust and continues even after encountering an exception.

Actual Results

AuditLogSaveThread is no longer running. Example of the idle thread:

"AuditLogSaveThread" #37 daemon prio=5 os_prio=0 tid=0x00007fc79bb90000 nid=0x1231 waiting on condition [0x00007fc78dffa000]
   java.lang.Thread.State: WAITING (parking)
	at sun.misc.Unsafe.park(Native Method)
	- parking to wait for  <0x00000000c6702bb8> (a java.util.concurrent.locks.AbstractQueuedSynchronizer$ConditionObject)
	at java.util.concurrent.locks.LockSupport.park(LockSupport.java:175)
	at java.util.concurrent.locks.AbstractQueuedSynchronizer$ConditionObject.await(AbstractQueuedSynchronizer.java:2039)
	at java.util.concurrent.LinkedBlockingQueue.take(LinkedBlockingQueue.java:442)
	at com.atlassian.bamboo.persister.DefaultAuditLogService$LoggerThread.run(DefaultAuditLogService.java:125)

Workaround

Restart Bamboo