Bitcoind rpc emulation

[dakk]

Emulate the bitcoind rpc apis in order to let letchain usable for external softwares like lightning. First we implement RPC apis used by elements/lightning:

• estimatefees
• sendrawtx
• getrawblock (hash)
• getblockcount
• getoutput (height, txnum, outindex)
• getblockhash (height)
• gettxout (txid, outindex)