influx_inspect walks the data and wal directories building a list of
files to export. It then opens, reads, and exports each. If the file was
deleted between the time it was added to the list and the time the
inspect tool attempts to read it, the file is now skipped without
emitting an error.
This switches all the interfaces that take string series key to
take a []byte. This eliminates many small allocations where we
convert between to two repeatedly. Eventually, this change should
propogate futher up the stack.