insights: sample on overflow

Insights events are now statistically sampled when they overflow the
builtin buffer size.
Add flatten util to flatten nested maps

Ex: {a: 5, b: {c: true, d: 7}} -> {a: 5, 'b.c': true, 'b.d': 7}

This will be used for sending agent settings to the collector, which
expects a flattened settings map for browser monitoring settings.

The test is in integration so I can use tap...
config: report local agent settings to collector

This change sends agent settings to the collector both at connect time
(so only local settings), and after any configuration change (so
including server-side settings).
mysql-service: Use mkdirp in case the dbpath already exists
docs: updated for new version
release: published 1.5.4