Сherry-pick rests of upstream commit ffc20e4 (subtask of #174)

[leha-bot]

The remaining things are:

• [ ] Create new data_flags;
• [ ] Update references to new data_flags;
• [x] Create new data_peer;
• [ ] Drop structs.h and structs.cpp.

Related commit is telegramdesktop/tdesktop@ffc20e4. Subtask of #174.

[leha-bot]

data_peer has been implemented in commit 26d6594ef276fd3cef30ba49b6b46ccecfe0238f