Add metric for fullnode/drone panics

[mvines]

Fixes #651

Eg:

$ influx -host metrics.solana.com -ssl -database scratch -execute 'SELECT * FROM "scratch"."autogen"."panic" WHERE time > now() - 10m'
name: panic
time                location                  message                                                                                                                                                 thread
----                --------                  -------                                                                                                                                                 ------
1531849375818000000 src/bin/fullnode.rs:24:22 panicked at 'oops, test panic', src/bin/fullnode.rs:24:22                                                                                                           ?
1531849585120000000 libcore/result.rs:945:5   panicked at 'called `Result::unwrap()` on an `Err` value: Os { code: 48, kind: AddrInUse, message: "Address already in use" }', libcore/result.rs:945:5 main

[mvines]

Not quite sure what 1.6 panics are but the graph is nice regardless: