Issue #393: add circuit breaker for network failures

[magiconair]

This patch adds a circuit breaker monitor which issues route table fragments to delete routes for which the circuit breaker is open.

The current implementation supports only the HTTP path. TCP and WS will follow.

The integration test isn't testing the behavior automatically yet but can be used to eyeball the behavior. This needs to be improved.

Also missing is the code to GC the circuit breakers that aren't tripped over time but that should be easy to implement in this code.

Fixes #393