lnd+config: add ability to obtain blocking and mutex profiles

In this commit, we add the ability to obtain blocking and mutex
profiles. The blocking profile will show which goroutines are
consistently blocked on synchronization primitives like channels, or
I/O. The mutex profile will show which mutexes are very contested.

The blocking profile can be enabled with a new arg: `--blockingprofile`.
The mutex profile can be enabled with a new arg: `--mutexprofile`. These
are both ignored if the profile port isn't set.

Activating these profiles requires the caller to pass in a sampling
rate. For now I've set it just to `1` to test things out. Unfortunately
documentation is rather scarce, so there aren't any good guides re what
these values should be set to. AFAICT, these add more overhead than the
other prowling options, so they shouldn't necessarily be enabled
persistently in production.

config.go

@@ -337,6 +337,9 @@ type Config struct {
 
 	Profile string `long:"profile" description:"Enable HTTP profiling on either a port or host:port"`
 
+	BlockingProfile int `long:"blockingprofile" description:"Used to enable a blocking profile to be served on the profiling port. This takes a value from 0 to 1, with 1 including every blocking event, and 0 including no events."`
+	MutexProfile    int `long:"mutexprofile" description:"Used to Enable a mutex profile to be served on the profiling port. This takes a value from 0 to 1, with 1 including every mutex event, and 0 including no events."`
+
 	UnsafeDisconnect   bool   `long:"unsafe-disconnect" description:"DEPRECATED: Allows the rpcserver to intentionally disconnect from peers with open channels. THIS FLAG WILL BE REMOVED IN 0.10.0"`
 	UnsafeReplay       bool   `long:"unsafe-replay" description:"Causes a link to replay the adds on its commitment txn after starting up, this enables testing of the sphinx replay logic."`
 	MaxPendingChannels int    `long:"maxpendingchannels" description:"The maximum number of incoming pending channels permitted per peer."`