[Azoth Murm] Refactored pending messages handling for #1379.
[Azoth Murm] Moved friends fields to a const for #1379.
[Azoth Murm] Refactored users parser into ParseUsers for #1379.
[Azoth Murm] VkConnection::GetUserInfo() for #1379.
[Azoth Murm] Properly handle messages from unknown users.

Request user info for the unknown user, create the entry and re-try
handling the message.

Fixes #1379.