[leo_storage] Suppress error logs on start

[mocchira]

Log lines like below occasionally get appeared on leo_storage start.

[W]     storage_3@127.0.0.1     2017-12-29 11:32:23.923 +0900   1514514743      null:null       0       lager_error_logger_h dropped 11 messages in the last second that exceeded the limit of 500 messages/sec

This can happen in case ops(operation per second) against error_logger exceeded error_logger_hwm.
I think many INFO level messages yielded on startup cause ops to exceed the threshold so it may be good to raise the default (current: 500) in leo_storage.conf however it's difficult to choose the optimal number as the higher value error_logger_hwm is set, the higher odds leo_storage can get stuck.

[mocchira]

rabbitmq/rabbitmq-server#1472 this solution (raising the error_logger_hwm only during the boot then setting the original value back once the boot finished) can be applied to this issue.