Increase max length of processed text to ~16MB

I'm creating some scripts that outputs more than 64KB, which is the current size limit of text checks (see ZBX-952). I would suggest to increase this limit to 16MB to match the database field capacity, at least in mysql, which is of type mediumtext (2**24-1 bytes, see http://dev.mysql.com/doc/refman/5.0/en/storage-requirements.html)