Log chain calls in ChainMonitor, reducing logs in ChannelMonitor

Log chain calls in ChainMonitor, reducing logs in ChannelMonitor

For users with many ChannelMonitors, we log a large volume per
block simply because each ChannelMonitor lots several times per
block. Instead, we move to log only once at the TRACE level per
block call in ChannelMonitors, relying instead on a DEBUG level
log in ChainMonitor before we call any ChannelMonitor functions.

For most users, this will reduce redundant logging and also log at
the DEBUG level for block events, which is appropriate.

Fixes #980.