Add LogglyBatchAppender to group log messages sent to Loggly HTTP API

Here is a proposal for a LogglyBatchAppender to group/batch log messages and asynchronously send them to Loggly HTTP API.
Batched log messages are temporarily stored in byte buffers and a daemon task sends the log message to Loggly.

Configuration is described in the JavaDoc of the LogglyBatchAppender.
An AbstractLogglyAppender is introduce to mutualize code between existing sync LogglyAppender and async LogglyBatchAppender.