The initial message sync has to fetch potentially large amounts of data
and insert that into the database. To work around limitations with sync
requests triggered by web requests the process had already been made
interruptable and resumable. This means we never insert all the data
right away. Yet, the IMAP code fetched all UIDs before we capped it to a
maximum number of results per sync attempt. Depending on the mailbox
size this operation could require and allocate a lot of memory. On some
setup with lower memory limits, the process was aborted by the web
server due to a php memory exhaustion.

This patch modifies the IMAP code to optimize the memory usage by
limiting the amount of data that is fetched with each initial sync
attempt. The algorithm works as follows.

IMAP allows us to search in a range with a lower an upper bound UID.
While we know the highest known UID from the current cache values, we
can't derive the range for the next page from that as UIDs are not
continuous but might have holes due to deleted messages. If we assume
that messages of a mailbox are roughly distributed equally across the
assigned UIDs we can guess the max UID for the next range.
So we ask the server for min and max UIDs. The min or our known highest
UID is always the lower bound. Then we can calculate the distribution
rate from the min, max and number of messages and build the upper bound.
On everage this will fetch about the expected number of messages. It
could be more, but it could also be less. It shouldn't matter in most
cases.

Signed-off-by: Christoph Wurst <christoph@winzerhof-wurst.at>