Return hashes from getlatestblock

[adityapk00]

Return the Hash (along with the Height) for the getLatestBlock RPC

[adityapk00]

cc: @LarryRuane

[LarryRuane]

Concept ACK, let me do a little testing, and then I would like to merge this, thanks.

[LarryRuane]

Added a unit test. (Also there was something missing from .gitignore, fixed even though not related to this PR.)

[LarryRuane]

After this PR merged, @gmale had some concern that increasing the byte length of the reply, as this PR does, would significantly increase the overall bandwidth (since this gRPC is executed every few seconds, I think). It may have been better to add a new gRPC that does what thisGetLatestBlock does now with this PR, and leave GetLatestBlock unchanged (so that it returns only the height). But the current thinking is that the way GetLatestBlock is now (with this PR) is probably okay, so we'll leave it as is, and we can always revisit later.

One idea I proposed is to return a shortened hash (low-order 4 bytes only) because all the hash is used for is to detect a reorg by comparing it to the previous hash we received. This would mean there's a one in 4 billion chance a reorg wouldn't be detected, but even in this case, it would eventually (when the next block arrives).