See https://github.com/facebook/rocksdb/wiki/MultiGet-Performance

- Avoid unnecessary copying of prev outpoints

- When looking for both mempool and on-chain txos, accumulate the set of
  outpoints that remain to be looked up to avoid re-checking for them
  later again in the found set

- Refactored lookup_txos() to use lookup_txo() internally rather than
  the other way around, which was less efficient

- Lookup txos in mempool first, then on-chain

- ChainQuery::lookup_txos() now returns a Result instead of panicking
  when outpoints are missing

- Removed ChainQuery::lookup_avail_txos() and allow_missing, which are
  no longer neceesary

Following Blockstream#89 (comment)