Only sync CONDSTORE and QRESYNC on the initial download.

Only sync CONDSTORE and QRESYNC on the initial download.

In the midst of the imap_read_headers() refactor, I forgot to put this
guard on the /MODSEQ and /UIDSEQSET storage.

Because we don't deal with flag sync issues while the mailbox is open,
or when it closes, we only want to write those values to the header
cache during the initial download.

It makes no sense to perform all the header cache work if new messages
come into an open empty mailbox, so add a parameter to flag the
initial download, rather than check for msn_begin==1.