Add support for getblockcount RPC call

[mpguerra]

Motivation

Mining pools use the getblockcount RPC call.

Mining Pool Usage

s-nomp calls getblockcount from this code:

• https://github.com/s-nomp/node-stratum-pool/blob/d86ae73f8ff968d9355bb61aac05e0ebef36ccb5/lib/pool.js#L595

Requirements

https://zcash.github.io/rpc/getblockcount.html

Testing

• Snapshot the output of the RPC
• Test calling the RPC from the mining pool software
• Manually compare the zcashd and zebrad RPC output using https://github.com/ZcashFoundation/zebra/blob/main/zebra-utils/zcash-rpc-diff

[oxarbitrage]

This seems to be just the current tip of the blockchain but the rpc method description of the docs is not up to date: zcash/zcash#2735 (comment)

[mpguerra]

Hey team! Please add your planning poker estimate with Zenhub @arya2 @conradoplg @oxarbitrage @teor2345 @upbqdn