add function to check the refcounts for Mailbox and Message on a fid
use the full 64 bit of the qid.path, so we can use the full 32 bit for the id
instead of only maintaining refcount for the top message, msgincref() now
adds a reference to all its parent messages including self and top message.
then we can check in recursive delmessage() that all the parts have a zero
refcount.
remove the Fid.mtop field, it was never used.
make sure deletion and flag changes only affect the top message.
cachefree(): only look for top message in lru. sub-parts are never
added to the cache.
use the nparts field when reading sub-part of existing message, so
that we parse the index right in case the number of parts somehow
changed.
messages marked as Deleted but still in inbox should be written
to the index.
8e50556f14