Implement "getchains" RPC command to monitor blockchain forks.

Port over https://github.com/chronokings/huntercoin/pull/19 from
Huntercoin:  This implements a new RPC command "getchains" that can be
used to find all currently active chain heads.  This is similar to the
-printblocktree startup option, but it can be used without restarting
just via the RPC interface on a running daemon.
Convention for RPC results is to not sometimes return a field and sometimes not.

So returning an empty branch Array here would be better.

Is branch_len always the length of the branch array? If yes, then it is redundant and should be removed.