add blocksToMine to testnet miner

[Alrighttt]

This pull requests adds an optional parameter to the walletd binary's mine command to allow the user to specify how many blocks to mine before stopping the process.

This was required for Komodo Wallet's test suite. No issue if it is not merged, but I thought it may be appreciated.

[lukechampine]

I have no specific objection to this feature, but given what I've seen here and in https://github.com/SiaFoundation/walletd/commit/0b53f40a281c6561b9beb3868f51a5feb8431c79, my feeling is that your best bet is to either fork walletd and adapt it for your purposes, or create a separate standalone binary that interfaces with walletd as necessary. (For example, instead of adding a /utils/generateaddress/ endpoint, your binary could have an addr CLI command.) Thoughts?

[Alrighttt]

We intend to maintain our own fork with added utility, testing and debugging endpoints. I currently have no preference whether this is merged.

To give to some context, this week I'll be adding all the endpoints I find necessary to create, sign and broadcast HLTC-like transactions.

[n8maninger]

Added in #54