Support for custom profiles for serf agent

[cipher90]

Currently serf provides local, lan and wan profiles. Parameters in these profiles are somewhat conservative(for example ProbeTimeout can be 99 percentile of RTT. But it is set to 200ms in local although RTT is much less~ 1-2ms in local network).
This is an enhancement request to support providing custom profile parameters via a json file to serf so that failure detection can be controlled by operators(i.e fast detection vs conservative detection)

example:
--customProfileFile="custom.json"

cat custom.json
{
"ProbeTimeout":"200 * time.Millisecond",
"SuspicionMult" :2,
"ProbeInterval" : "200 * time.Millisecond"
}

Thanks.

[amanchourasiya]

This seems like a nice solution, because once I had to tune up these values manually for faster failure detection. It would me nice if we can make them configurable.