* Store our own keys in the keydb
The DirectKeyFetcher makes the assumption that you can always reach the key/v2/server endpoint of any server, including our own. We previously haven't bothered to store our own keys in the keydb so this would mean we end up making key requests to ourselves.
In the libp2p world as an example, self-dialling is not possible, therefore this would render it impossible to get our own keys.
This commit adds our own keys into the keydb so that we don't create unnecessarily (and maybe impossible) requests.
* Use golang.org/x/crypto/ed25519 instead of crypto/ed25519 for pre-Go 1.13
* Fall back to postgres when parsing the database connection string for a URI schema fails
* Fix behaviour so that it really tries postgres when URL parsing fails and it complains about unknown schema if it succeeds
