jamiaccount: return syncing conversations

This changes tends to simplify the client code to
detect syncing conversations. Note, this patch
introduces some breaking changes for the stored
convInfos, but this will be fixed when all devices
are up-to-date.

Changes:

+ getConversations() now returns conversations as soon
as the request is accepted.
+ getConversationMembers() return members for syncing
conversation (current account and the sender of the
request)
+ "convInfo" now stores a map<string, ConvInfo> to
avoid iterating over all conversations. This changes
the structure stored on the disk and sent during the sync.

GitLab: #591
Change-Id: I365875cda89c483bcf9c6ea3bd8222e6253773b1