Update acquaintances when addTalkers/deleteTalk/deleteTalker

Update acquaintances when receiving these notifications from direct4b.com:
- notify_add_talkers
- notify_delete_talk
- notify_delete_talker
Tests without accessing to direct4b.com, for the functions called when receiving the notifications above.
- Expose some functions in direct-hs and network-messagepack-rpc for testing.
New way to store acquaintances of Client:
- When receiving notify_add_talkers, the client can get only the user IDs of the new talkers: the client can't update acquaintances due to lack of the details (such as screen names) of the new talkers.
- In addition, direct4b.com doesn't seem to provide some API to get details of a user by the user ID anyway!
- To get correct and up-to-date information of acquaintances, I decided not to update acquaintances immediately after receiving notify_add_talkers: Instead, invalidate the cache of acquaintances when the client should get the details of new talkers after receiving notify_add_talkers.
- After invalidating the cache of acquaintances, getAcquaintances and modifyAcquaintances automatically call the MessagePack RPC function get_acquaintances to get correct and up-to-date information of acquaintances again then cache it.

iijlab / direct-hs