lei import: speed up kw updates for old IMAP messages
On a 4-core CPU, this speeds up "lei import" on a largish IMAP
inbox with 75K messages from ~21 minutes down to 40s.
Parallelizing with the new LeiImportKw WQ worker class gives a
near-linear speedup and brought the runtime down to ~5:40.
The new idx_fid_uid index on the "fid" and "uid" columns of
blob2num in mail_sync.sqlite3 brought us the final speedup.
An additional index on over.sqlite3#xref3(oidbin) did not help,
since idx_nntp already exists and speeds up the new ->oidbin_exists
internal API.
I initially experimented with a separate "lei import-kw" command
but decided against it since it's useless outside of IMAP+JMAP
and would require extra cognitive overhead for both users and
hackers. So LeiImportKw is just a WQ worker used by "lei import"
and not its own user-visible command.
v2: fix ikw_done_wait arg handling (ugh, confusing API :x)