Add recent operation tracking and API call for recent registrations

The chain database now retains in RAM the 20 most recent operations for
all operation types. I've also added an API call to get the recent
account registrations.
Merge branch 'develop' of github.com:BitShares/bitshares_toolkit into develop
Add new security level, grey, for use when syncing chain

Previously the backend reported the state as red when syncing. Now it
reports it as grey.