sqlite-lib: Disallow :memory: connection

[lukedirtwalker]

Using just :memory: has some issues as pointed out by mattn/go-sqlite3#204 and https://github.com/mattn/go-sqlite3#faq, so our lib should fail out if being initialized with :memory: only.

[jiceatscion]

The problem seems to be (after reading the referenced issues), that configuring the DB uri to be ":memory:", as opposed to "file::memory:?cache=shared" (or a couple of equivalent expressions) leads to surprising behaviors that are typically not what the user wants. That is: should the same DB be opened more than once in a given process, each instance is independent (and there are also some potential data races). So, to prevent users from shooting themselves in the foot, we should disallow it. "file::memory:?cache=shared", on the other hand, would be fine. Note that issue 304 warns about the performance penalty of opening the same DB twice, but that's not necessarily a severe issue for us.

[jiceatscion]

In any case, we should document what configured values are acceptable.