CDAP-62: Log Saver process throws OOM exception. 

Currently there is no limit on the number of events that can be in the in memory messageTable which could cause the Log Saver to throw OOM.

Fix is to maintain the in memory window of events containing fixed number of buckets (controlled by config variable "log.saver.event.max.inmemory.buckets") in the messageTable. 

If the in-coming event falls out of the window then sleep till the window moves forward and include the event.  
By default, the buffer keeps eight buckets of events in memory. Each event bucket contains logs generated for one second. The Log Saver process is limited to 1GB of memory. When it is expected that logs exceeding these settings will be produced, for example, greater than 1GB of logs generated in eight seconds, increase the memory allocated to the Log Saver or increase the number of Log Saver instances. If the cluster has limited memory or containers available, you can choose instead to decrease the number of in-memory event buckets. However, decreasing the number of in-memory buckets may lead to out-of-order log events.