[SPV] Implement MemPool

[Warchant]

• Make SPV listen for all transactions in a network #323
• On new VBK block, pull full block body #324
  • use it to update info about mempool transactions: confirmations=-1 if tx status is unknown, 0 if in mempool, positive if in blockchain. e.g. 1 = tx is in a tip.
  • do not store block body, save only headers
• Determine current address signature index: #325
  • use ledger proof to get current blockchain sigindex
  • lookup all address txes in local mempool and determine highest mempool sigindex
  • drop all address txes in MemPool which have sigindex <= blockchain sigindex
  • drop all address txes in MemPool whose sigindex is after a gap. Blockchain: 100, mempool has 101, 102, 105. Drop tx with sigindex 105.
• Track tx status #326
  • statuses: unknown (remote peer doesn't know about this tx), in mempool, in block(S).
  • use block bodies to determine tx data (in a block, etc)
  • track: confirmations, containing blocks (based on full block data)
  • use "tx info request" for ghost txes - those txes that do not appear in blocks for N=100 blocks.
• Calculate min/max/avg tx fee in mempool #327
  • provide convenient getter in SPV
  • Let user choose between max/avg/min/fixed fee per byte in application.conf #328

Fixes https://github.com/VeriBlock/vbk-ri-btc/issues/603
Fixes https://github.com/VeriBlock/vbk-ri-btc/issues/606
Fixes VeriBlock/vbk-ri-btc#601