rpcserver: add router synced to synced_to_chain flag
rpcserver: add router synced to synced_to_chain flag

The router has a lot of work to do for each block. So it might be
possible that it isn't yet up to date with the most recent block,
even if the wallet is. This can happen in environments with high CPU
load (such as parallel itests). Since the `synced_to_chain` flag in
the response of this call is used by many wallets (and also our
itests) to make sure everything's up to date, we add the router's
state to it. So the flag will only toggle to true once the router was
also able to catch up.

aakselrod/lnd,aakselrod/lnd