Implement stalling logic during initial sync

[torkelrogstad]

Right now the enforcer does initial sync much faster than Core does. As-is the enforcer crashes if we get ahead of Core. Should implement stalling logic that handles this gracefully.

[torkelrogstad]

Could be an idea to wait to kick off our sync until Bitcoin Core has completed header sync. Could determine this by doing getpeerinfo and checking what the best peer height is, waiting until getblockchaininfo has a headers field that is the same as the best peer height OR IBD finishes (to prevent stalling forever)