Feat/bitcoind backend (#33)

* wip add bitcoind backend

* wip fix timeout

* allow host/port from options

* wip call scanblocks from add not ls

* filter false positives

* wip change to use new scanblocks rpc, doesn't build

* use bdk wallet_rpc custom example, builds now

* remove print statements

* fix agressive locking and don't rescan every block. we still rescan if the same wallet is added again

* don't use local deps

* return more helpful error when wallet in datastore is invalid

* even better error message

* fix chain initialization connect error

* cleanup

* fix brpc_user error msg and cleanup logging

* more cleanup

* more cleanup

Author: chrisguida