Support multiple sessions for each user

Closes #3

See merge request !32

Do not have more than one passdb request in-flight for a user

See merge request !34

Prevents us from making 4-5 concurrent requests (or fallbacks to
decryption from database) for the same user when clients like
Thunderbird connect.

Fix dovecot-keylookupd logging to log a single line per request

See merge request !33

Also provide backwards compatibility for API clients that do not set
the session_id attribute, in which case the server reverts to the old
behavior (no support for multiple concurrent sessions).

Breaking API change, adding a "session_id" field to Open and Close
calls.

Multiple sessions can be attached to the same decrypted key, each
expiring (or being closed) independently: the key will only be dropped
when the last session terminates.

Update module github.com/lib/pq to v1.10.7

See merge request !18

Update module github.com/coreos/go-systemd/v22 to v22.5.0

See merge request !24

Update git.autistici.org/ai3/go-common digest to 0630401

See merge request !25

Update module github.com/go-sql-driver/mysql to v1.7.0

See merge request !30

Update module github.com/mattn/go-sqlite3 to v1.14.16

See merge request !28

Update git.autistici.org/id/go-sso digest to a98dfc7

See merge request !31

Using a *yaml.Node does not work.

Update git.autistici.org/id/go-sso digest to 8a1845d

See merge request !22

Update golang.org/x/crypto commit hash to 5ea612d

See merge request !9

Update module lib/pq to v1.10.0

See merge request !11

Update module coreos/go-systemd/v22 to v22.2.0

See merge request !10