Prune: Support noncontiguous block files

In some corner cases, it may be possible for recent blocks to end up in
the same block file as much older blocks.  Previously, the pruning code
would stop looking for files to remove upon first encountering a file
containing a block that cannot be pruned, now it will keep looking for
candidate files until the target is met and all other criteria are
satisfied.

This can result in a noncontiguous set of block files (by number) on
disk, which is fine except for during some reindex corner cases, so
make reindex preparation smarter such that we keep the data we can
actually use and throw away the rest.  This allows pruning to work
correctly while downloading any blocks needed during the reindex.

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