Improve MySQL performance

[jorainer]

Using EnsDb databases in a MySQL server has currently some performance issues. The reason are the indices and identifiers that are character. SQLite has no problem with that, MySQL struggles when it comes to join tables.

Needed changes:

• [x] introduce integer internal IDs (e.g. internal_gene_id). Done in https://github.com/jotsetung/ensembldb/commit/2e67d497efd9fc7a45c6e1fca3deb21d1d481141
• [x] this requires to amend the joins for MySQL using the above columns/indices. i.e. the join has to be performed differently for SQLite or MySQL backend. Done in https://github.com/jotsetung/ensembldb/commit/473139eb19e9e48e3617f04475fbb38fc2adb4bd

Why not introducing internal IDs also for the SQLite backend: no performance difference between character and integer-based indices/columns in SQLite. Including them would only blow the database up.