feat(mine): Add an internal Zcash miner to Zebra #315

Job	Run time
Check if cached state disks exist for Mainnet / Check if cached state disks exist	0s
Build CI Docker / Build images	0s
Generate checkpoints mainnet / Run checkpoints-mainnet test	0s
Zebra tip JSON-RPC / Run fully-synced-rpc test	0s
lightwalletd tip send / Run lwd-send-transactions test	0s
Zebra checkpoint update / Run sync-past-checkpoint test	0s
get block template / Run get-block-template test	0s
lightwalletd tip / Run lwd-full-sync test	0s
Zebra tip update / Run update-to-tip test	0s
lightwalletd GRPC tests / Run lwd-grpc-wallet test	0s
lightwalletd tip update / Run lwd-update-sync test	0s
submit block / Run submit-block test	0s
	0s

Provide feedback