Implement cross-signing proposal

[uhoreg]

Implement server-side part of matrix-org/matrix-spec-proposals#1756. Part of element-hq/riot-meta#222.

• manage master keys
• store and retrieve local signatures
• make sure signatures are properly filtered
• notify users when new signatures need to be fetched
• federate signatures
• sytest everything
• unit test everything

[uhoreg]

Main remaining work is cleaning up the code. We also need to decide whether to use the three-key system or the two-key system, but changing that should not take too long.

[Thatoo]

Do you think cross-signing will be implemented in synapse 1.0?