MySQL: correct LONGTEXT and LONGBLOB limits from 2GB to their true 4GB
MySQL: schema.rb now includes TEXT and BLOB column limits.
Include default column limits in schema.rb

Allows :limit defaults to be changed without pulling the rug out from
under old migrations that omitted :limit because it matched the default
at the time.
Speed up schema dumper tests

Dump the standard schema once instead of redoing it per test