deadpool-sqlite: Obtain read-only connection

[cvengler]

It would be very sweet if deadpool-sqlite would offer a way to obtain a read-only or rw connection during runtime.
I have a few places in my code, where I have transactions that shall never get upgraded to a write connection; this can be ensured nicely with SQLites read-only connection opening.

[bikeshedder]

This is related to OpenFlags:

• Add support for OpenFlags and vfs parameter to deadpool-sqlite #214

I think one would want to create two pools for this:

• A read-only pool
• A read-write pool

The latter should probably be generated with a much lower pool size as SQLite doesn't scale very well with concurrent writes anyways.