Make storage.mysql-engine.max-size independent on max_allowed_packet

Summary:
I like systems that just work. It is possible to store files larger than max_allowed_packet in MySQL and we shouldn't demand it.

It also fixes a problem when file was smaller than `storage.mysql-engine.max-size` but its escaped version was larger than `max_allowed_packet`.

Test Plan: Reduced the size to 5e4, uploaded 90 kB file, checked the queries in DarkConsole, downloaded the file.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D3392

src/docs/configuration/configuring_file_storage.diviner

@@ -41,8 +41,7 @@ Builtin storage engines and information on how to configure them.
 MySQL storage is configured by default, for files up to (just under) 1MB. You
 can configure it with these keys:
 
-  - ##storage.mysql-engine.max-size##: Change the filesize limit. Note that
-    this must be smaller than 'max_allowed_packet' on the server. Set to 0
+  - ##storage.mysql-engine.max-size##: Change the filesize limit. Set to 0
     to disable.
 
 For most installs, it is recommended you configure local disk storage below,

src/docs/configuration/configuring_file_upload_limits.diviner

@@ -41,8 +41,7 @@ limit uploads are:
     configured storage engine which can accept it. Phabricator should give you
     useful errors if any of these fail.
     - **MySQL Engine**: Upload size is limited by the Phabricator setting
-      `storage.mysql-engine.max-size`, which is in turn limited by the MySQL
-      setting `max_allowed_packet`. This often defaults to `1M`.
+      `storage.mysql-engine.max-size`.
     - **Amazon S3**: Upload size is limited by Phabricator's implementation to
       `5G`.
     - **Local Disk**: Upload size is limited only by free disk space.