[WIP] MySQL prepared statements

[CromFr]

Prepared statements provide a safer (no more string escaping required) and more efficient (if identical statements are called frequently) way of running SQL queries.

This PR defines the prepared statements API for SQL plugins, and implements support for the xp_mysql plugin.

TODO:

• Check that returned strings are not changed on SQL fetch (i.e. check if it's strcpy-ed or if it retains the pointer to the original data)
• Check results client-side caching (I'm not sure it's enabled)
• Try improving performance (currently it's only ~10% faster)
• Implement for xp_sqlite? (should be fairly straightforward)