New driver options:
- `splunk-gzip` - gzip compress all requests to Splunk HEC
(enabled by default)
- `splunk-gzip-level` - change compression level.
Messages are sent in batches by 1000, with frequency of 5 seconds.
Maximum buffer is 10,000 events. If HEC will not be available, Splunk
Logging Driver will keep retrying while it can hold messages in buffer.
Added unit tests for driver.
Signed-off-by: Denis Gladkikh <denis@gladkikh.email>
