Add RPC endpoint for uploading a mined block

[jcnelson]

This one's pretty straight-forward: takes a block, stores it if the node doesn't have it yet, and (in doing so) causes it to notify its peers that the block is available if it wasn't known already. We'll need this for mining.

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[stale]

This issue has been automatically closed. Please reopen if needed.

[blockstack-devops]

This issue has been automatically locked since there has not been any recent activity after it was closed. Please open a new issue for related bugs.