Simplify request handling

Signed-off-by: Zack Cerza <zack.cerza@inktank.com>
Set serializable isolation level by default

Signed-off-by: Zack Cerza <zack.cerza@inktank.com>
Add test for lock_many race condition

This test requires paddles to be running in a multi-worker mode, or else
it is skipped.

Signed-off-by: Zack Cerza <zack.cerza@inktank.com>
Add teardown method

Signed-off-by: Zack Cerza <zack.cerza@inktank.com>