Set default Delta value to 3 seconds (#191)

Update the default delta value to 3 seconds and document the rationale
behind it.

The defaults may change in the future once concrete measurements on a
live network are performed.

gpbft/options.go

@@ -6,7 +6,7 @@ import (
 )
 
 var (
-	defaultDelta                = 2 * time.Second
+	defaultDelta                = 3 * time.Second
 	defaultDeltaBackOffExponent = 1.3
 )
 
@@ -35,7 +35,13 @@ func newOptions(o ...Option) (*options, error) {
 }
 
 // WithDelta sets the expected bound on message propagation latency. Defaults to
-// 2 seconds if unspecified. Delta must be larger than zero.
+// 3 seconds if unspecified. Delta must be larger than zero.
+//
+// The default of 3 seconds for the value of Dela is based on the current EC
+// propagation time (i.e. round-trip time of 6 seconds) as the initial
+// propagation timeout. The actual propagation time may be significantly lower.
+// However, until further measurements confirm it a 3 second value for delta
+// seems reasonable.
 //
 // See: https://github.com/filecoin-project/FIPs/blob/master/FIPS/fip-0086.md#synchronization-of-participants-in-the-current-instance
 func WithDelta(d time.Duration) Option {