sqlitepool: new package, implements a pool

[crawshaw]

This is intended as a replacement for our internal sqlitedb package pool and its TxWrapper.

It needs a little more machinery to be dropped in that's not covered in this PR, in particular:

• helper functions that mimic ExecContext/QueryContext/QueryRowContext to make replacing the existing uses of TxWrappers easy
• a little more Tracer plumbing, so we can get SQL query exec times out in the same format we are used to them now
• a few other missing counters

This PR implements the Tx/Rx object split we've discussed a few times. It will take a bit of work to make good use of that internally. But I figured it's worth getting in now.

After we've replaced the current pool, I plan to have the code generator for queries work directly on top of this pool.

Any other features you want to see in a pool?

[crawshaw]

This PR now includes a second commit filled with basic implementations of the Exec/QueryRow/Query, like database/sql.

Long-term I'm hoping to replace these with generated access to statements (like sqlc, but without the allocations), but this lets us get sqlitepool into our server with mostly-mechanical perl one-liners.

[crawshaw]

Ready for review.

See it in action here: https://github.com/tailscale/corp/pull/5720