Drop last bits of rescan as its too complicated to be worth having

Drop last bits of rescan as its too complicated to be worth having

Previously, we had a concept of "rescaning" blocks when we detected
a need to monitor for a new set of outputs in future blocks while
connecting a block. In such cases, we'd need to possibly learn about
these new spends later in the *same block*, requiring clients who
filter blocks to get a newly-filtered copy of the same block. While
redoing the chain access API, it became increasingly clear this was
an overly complicated API feature, and it seems likely most clients
will not use it anyway.

Further, any client who *does* filter blocks can simply update their
filtering algorithm to include any descendants of matched
transactions in the filter results, avoiding the need for rescan
support entirely.

Thus, it was decided that we'd move forward without rescan support
in #649, however to avoid significant further changes in the
already-large 649, we decided to fully remove support in a
follow-up.

Here, we remove the API features that existed for rescan and fix
the few tests to not rely on it.

After this commit, we now only ever have one possible version of
block connection transactions, making it possible to be
significantly more confident in our test coverage actually
capturing all realistic scenarios.