lei_mail_sync: rely on flock(2), avoid IPC

lei_mail_sync: rely on flock(2), avoid IPC

Since 44917fdd24a8bec1 ("lei_mail_sync: do not use transactions"),
relying on lei/store to serialize access was a pointless endeavor.

Rely on flock(2) to serialize multiple writers since (in my
experience) it's the easiest way to deal with parallel writers
when using SQLite.  This allows us to simplify existing callers
while speeding up 'lei refresh-mail-sync --all=local' by 5% or
so.