1.6.4.3

Maintenance release in preparation for 2.0 release.
CORE - Backport new testnet from 2.0 branch, this is required in order to beta test 2.0 upgrades correctly.
Specify the desired testnet chain using the format : e.g. S1505211751:20 (Scrypt with 20 second block target) or C1505211751:10 (City hash with 10 second block target). For 'official' testnet chain specifiers visit https://github.com/Gulden/gulden-official where the latest chains will always be listed.
CORE - Introduce some corrections/changes to all mutex related code in an attempt to fix some occasional RPC freezes that are intermittently occurring.
CORE - Pre-emptively patch/fix against a potential but unlikely DDoS attack that could be used to try temporarily make specific nodes inaccessible.
CORE - Fix a minor issue with encrypted non-HD wallets where it was necessary to input the password on every GuldenD restart.
MISC - Update all nocks calls to use the new nocks.com API.
MISC - Clean up some unnecessary logging to keep log files small.
MISC - Periodic update of checkpoints and other hardcoded wallet information.
CMDFLAG - Fix issue with genkeypair command.