0.9.18 Race condition bugfix and new config option

1. Fixes #74 where buffer was not thread local and multiple threads were using same instance. Thanks to @thgau for reporting this.
2. Re. #77: adds configuration option to control how often log shipping agent wakes up to check if there is enough logs to be sent to Loki. What's more important is that default value of 1ms was changed to 10ms to reduce CPU usage.