multi: perform cross repo integer audit

In a few areas we do things like use a uint64 for a locktime, when in practice from Bitcoin's PoV a uint32 is all that's needed.

There're other areas where we mix signed and unsigned integers as well, and need to cast back and forth between them.

This also trickles down into the database: sqlite doesn't actually support `uint64` type natively. Also depending on what top-level types you use, `sqlc` may translate them into unexpected integers.

When should do a general audit/run-thru of all the integer usage within the project, particularly as they interact w/ the general database structure, and also the SMT itself.

Related to #37