Allow RPC startup sync cancellation
Allow RPC startup sync cancellation

This change introduces additional checks of the context variable during header
and cfilter fetching when catching up to a remote dcrd node in the RPC sync
mode.  This results in more responsive process shutdown if the RPC syncer is
currently fetching headers from the node when shutdown is requested.

jrick/dcrwallet,decred/dcrwallet,jrick/btcwallet,jrick/btcwallet,jrick/dcrwallet,decred/dcrwallet