Redesign Prepared (Fixes #95, #97)

[Abscissa]

This intends to fix #95. It should also improve reliability as it also fixes #97.

Full description, including motivation, is in the included file ABOUT_PREPARED_V2.md

Tasks:

• Eliminate PreparedImpl.
• Eliminate Prepared.exec/Prepared.query. (Make them overloads of the regular exec/query functions.)
• Eliminate Prepared.release/Prepared.register.
• Eliminate Prepared's reference to a Connection.
• Eliminate mysql.prepared's dependency on mysql.connection (except when compiling unittests).
• Connection manages a list of the prepared statements registered on it.
• Implement internal Connection.release/Connection.register functionality.
• Create public Connection.release/Connection.register.
• Create public ConnectionPool.release/ConnectionPool.register (delayed for follow-up release).
• Automatically register Prepared the first time it's used on a Connection (if not already manually registered).
• Ensure a prepared statement cannot accidentally remain in the release queue after it's re-registered. (This shouldn't be possible anyway, but need to be sure)
• Temporary compatibility layer: prepareBackwardCompat.
• Test calling release during GC cycle. (blocked by Solve the prepared statement automatic-release difficulty #159)
• Implement Convenience exec/query overloads that do parameterized SQL #118 (delayed for follow-up release).
• Prune dead imports.
• Write "Migrating to v2.0.0" documentation.
• Double-check for sufficient unittesting.
• Double-check all relevant documentation.

[Abscissa]

Just waiting for travis-ci approval now...