Lock olm sessions between encrypting and sending

Fix deserializing media info

Bump version to 0.7.9

Bump version to 0.7.8

Bump version to 0.7.7

Bump version to 0.7.6

Bump version to 0.7.5

Bump version to 0.7.4

Bump version to 0.7.3

Allow passing extra parameters to SQLAlchemy's create_engine

Closes mautrix#27