Fetch file data in chunks for downloads

MySQL has a limit on the maximum amount that can be transferred in one
statement. It's the max_allowed_packet setting. The value of this setting
will be the approximate upper limit of attachments that can be handled by
the database given the current access model for osTicket.

The issue came up for attachment uploads and was corrected, so that uploads
are chunk inserted into the database. Downloads, however, were forgotten.
Strangely, it took quite a bit of debugging to track down the problem.

This patch corrects attachment downloads by fetching 256kB chunks of the
attachment at a time and sending them directly to the client. This will also
overcome PHP's memory limit which would be the second-level blocker of
attachment sizes. Lastly, the AttachmentFile::getData() method is simulated
using output buffering. This will provide the same access as the previous
getData() method; however, it is still subject ot PHP's memory limits.
Fix truncating of attachments on upload

MySQL is kind enough to quietly truncate the filedata field when attempting
to CONCAT data beyond the size of max_allowed_packet. The simplest fix is to
automatically adjust the max_allowed_packet to the size of the file being
uploaded plus some extra. See MySQL bugs #22853, #34782, and #63919 for more
discussion on the issue.

The max_allowed_packet variable default to 1M, but is expandable to 1G.
Therefore, the fixed limit of attachments for osTicket will be 1G, since it
would be impossible for MySQL to append data after that mark. *Sigh*
Merge branch 'develop', remote-tracking branch 'upstream' into issue/chunk-fetch-attachments
Add chunked file data support

This will remove the upper limit of BLOB sizes imposed by MySQL with the
max_allowed_packet setting completely.

This adds a new table %file_chunk which will contain the filedata in smaller
chunks (256kB). It also includes a new class, AttachmentChunkedData, which
will handle reading and writing the data, abstracting away the chunks.

This is done by migrating data from the %file table to the %file_chunk
table. One must beware that this must safely (the migration that is) plug
into the both the live osTicket developers as well as the users doing a full
upgrade from osTicket-1.6*. For this, the AttachmentFile::save method was
patched to use the new AttachmentChunkedData class to write the attachment
data to the database in chunks. That is, the migrater will use the new code
on the major upgrade and bypass the filedata column of the %file table
altogether. Therefore, the patch associated with this commit will not
migrate any data for the major upgrade.

For developers doing incremental upgrades, the patch included in this
commit will transfer the data from the %file data to the new %file_chunk
table by chunking it. As written, only the first 16MB of the attachment is
migrated. This could easily be adjusted, but it seems like a reasonable
limit for now.
Change attachment migration process for upgrade

Don't migrate attachments as part of the 1.6* major upgrade. Wait unit
transitioning to schema signature dd0022fb, where the new table for
file_chunks is created.

Incremental developers will already have the columns dropped from the
%ticket_attachments table and will receive an error for the cleanup script,
but the upgrader is configured to ignore errors from the cleanup scripts.
Simplify interface to AttachmentChunkedData
`isdeleted` column does not exist
AttachmentChunkedData::read no longer uses $length
Merge pull request #263 from greezybacon/issue/chunk-fetch-attachments

Reviewed-By: Peter Rotich <peter@osticket.com>