Change log levels on a running node

[mcamou]

Is your feature request related to a problem? Please describe.

Debug logs are useful for debugging issues but should not be enabled all the time due to performance and storage issues. When a node is malfunctioning and we want to enable debug logs, we need to restart it, which might clear up the issue that was causing it to misbehave and we'd need to wait until it recurs.

Describe the solution you'd like

Having some way of changing log levels on a running node would be very good. We should use the same mechanism as we use in #649 to enable/disable pprof.