This commit moves makeTestDB to db.go and exports it so that we'll be
able to use this function in other unit tests to make them testable with
etcd if needed.
In this commit, we modify the WitnessCache's
AddPreimage method to accept a variadic number
of preimages. This enables callers to batch
preimage writes in performance critical areas
of the codebase, e.g. the htlcswitch.
Additionally, we lift the computation of the
witnesses' keys outside of the db transaction.
This saves us from having to do hashing inside
and blocking other callers, and limits extraneous
blocking at the call site.
In this commit, we add the WitnessCache sub-storage system of the
greater database. The WitnessCache is a persistent cache of all
witnesses we’ve encountered on the network. We’ll use this cache to
share any on-chain discoveries between active channels. Eventually
we’ll also use this to enforce the variant that a preimage is only to
be used ONCE on the network.
