Prepare 0.12.1-SNAPSHOT
AUTHORS: Fix Martin's name.
Suppress noisy log line during signing.
Extend exception message to help track down flaky test.
Suppress noisy log line that is duplicated by LocalTransactionSigner.
Fix duplicate 'BTC' in log message.
PeerGroup improvements:

1) Don't hold the PeerGroup lock across DNS discovery, otherwise the API is high latency in this period of startup. Fixes issue in Lighthouse where the UI would not appear until DNS resolution had completed.

2) Don't backoff peers that failed due to a first-time connection error.

3) If an IPv6 peer fails to connect due to a NoRouteToHostException, don't try any more IPv6 peers in future.
Add/Remove wallets to the connected peers as they are added/removed
to/from the PeerGroup
Add javadoc comment for `VersionedChecksummedBytes.toString()`.
Add missing JavaDocs for ExchangeRate.
Fix one unexpected case of fiatToCoin overflow and add tests.
Added check to determine if the DB connection has been closed. If yes, then get a new connection.
Sort keys by age when printing them. Also fix a generics warning.
Replace two occurences of pom versions with ${project.version}. It makes incrementing the version a little bit less painful.
Improve Coin range check to cope with Long.MIN_VALUE correctly
When deserializing client payment channel state, if there was an existing close transaction, it was deserialized from wrong data.
Key rotation: remove the enabled setting. It's no longer useful and defaulted to off, which is dangerous and can lead to bugs.
Key rotation: construct new HD chain based on the oldest possible key, a la upgrade, with a fresh random HD chain only being created if all random keys are rotating.
Rename maybeDoMaintenance to doMaintenance and add a bit more docs.
Add maybeDoMaintenance back as a deprecated alias.