Migrate to BDK wallet

[raphjaph]

The ord/bdk branch is where all the work is collected.

TODO:

• make sure we can support multiple accounts without reentering key
• keep open the possibility of a wallet password
• make sure we can support multisigs in the future
• allow configuring lookahead (gap limit), setting?
• should we publish our persister to get review?
• should we pay a BDK dev to review/audit our code?
• think very hard about the database schema before final merge
• can we use the new descriptor format for combined external and internal (<0;1>)?
• add test for timestamp restore
• make sure that if we drop the wallet without persisting, we either panic, or we try to persist

Notes

• what should the format for importing/restoring a wallet from a descriptor be? The same as before?
• how should we handle the rawtr(...) commit output backups/recovery keys?
• for the commit output could we derive the untweaked key pair from the descriptor?

Tests

• test that you get error when ord has wrong network

TODO after merge into master

• rename TestServer to Ord
• rename run_and_deserialize_outptut to run_and_deserialize_stdout
• use strongly-typed Snafu errors for all wallet errors