healthcheck: monitor access to chain backend

Add a new health check package which will periodically poll health
check functions and shutdown if we do not succeed after our set number
of attempts. The first check that we add is one for our chain backend,
to ensure that we are connected to a bitcoin node.

sample-lnd.conf

@@ -400,3 +400,21 @@ litecoin.node=ltcd
 ; sweep funds if a breach occurs while being offline. The fee rate should be
 ; specified in sat/byte, the default is 10 sat/byte.
 ; wtclient.sweep-fee-rate=10
+
+[healthcheck]
+; The number of times we should attempt to query our chain backend before
+; gracefully shutting down. Set this value to 0 to disable this health check.
+; healthcheck.chainbackend.attempts=3
+
+; The amount of time we allow a call to our chain backend to take before we fail
+; the attempt. This value must be >= 1s.
+; healthcheck.chainbackend.timeout=10s
+
+; The amount of time we should backoff between failed attempts to query chain
+; backend. This value must be >= 1s.
+; healthcheck.chainbackend.backoff=30s
+
+; The amount of time we should wait between chain backend health checks. This
+; value must be >= 1m.
+; healthcheck.chainbackend.interval=1m
+