account manager: set private key on first load

src/jamidht/account_manager.cpp

src/jamidht/account_manager.h

src/jamidht/archive_account_manager.cpp

src/jamidht/archive_account_manager.h

src/jamidht/jamiaccount.cpp

src/jamidht/server_account_manager.cpp

src/jamidht/server_account_manager.h