Merge #842: Standalone module for creating a spend transaction

0f69411 spend: a nicer interface for providing fee informations (Antoine Poinsot)
990b153 commands: don't query unconfirmed coins when creating recovery tx (Antoine Poinsot)
c63a120 spend: don't use database's Coin type (Antoine Poinsot)
08ce0ad spend: update comment about create_spend behaviour (Antoine Poinsot)
0c395bb spend: document the create_spend function (Antoine Poinsot)
f3113ba commands: remove redundant output value check (Antoine Poinsot)
5894e78 spend: move tx size calc helper back to command module (Antoine Poinsot)
6ddda61 spend: make coin selection helpers private (Antoine Poinsot)
33be1ff commands: make create_recovery use the create_spend helper (Antoine Poinsot)
0523f00 commands: update next deriv index for any spend output address (Antoine Poinsot)
5d50155 spend: avoid direct access to our Bitcoin backend (Antoine Poinsot)
7c23812 spend: don't access the database in the PSBT creation function (Antoine Poinsot)
22f97e1 spend: let caller update next derivation index (Antoine Poinsot)
9fdb75c commands: split up spend transaction creation into its own module (Antoine Poinsot)

Pull request description:

  Based on top of #816, this introduces a new `spend` module with a helper to create a transaction spending coins from the wallet. It can be leveraged to create regular or recovery transactions, and also replacement for them. All the data structures used by the exposed spend creation function are contained with this module, in order to make it usable without a `lianad`-specific database and Bitcoin interface.

  This PR is structured in an incremental fashion. First we pull out the `create_spend_internal` method introduced in #816 into a standalone `spend` module, then we incrementally remove the cruft and the ties from the spend module to the other components.

ACKs for top commit:
  darosior:
    self-ACK 0f69411

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