Allow runtime modification of log levels

The node respects at least the following environment variables when determining how and what to log.

• TEZOS_EVENTS_CONFIG
• TEZOS_LOG
• LWT_LOG

Acceptable values and the intended effect of these variables is not documented. See #978

One limitation of implementing these features via environment variables is that any changes in value will only be reflected when the node is restarted. It would be nice to have control of this value while the node is running. See #1055 (closed)

Implement a feature to allow an authorized user to modify the node's log levels without requiring a restart.