[core] Fixed proper reporting of sending blocked state. Fixed API value for connect-non-blocking

[ethouris]

1. Collecting information about blocked sending (with none succeeded)
2. In non-blocking mode, this should result in reporting AGAIN error
3. In blocking mode, it should block until at least one socket is ready for sending, and then only report errors if none succeeded.

Additionally: the value returned in case of blocking-mode srt_connect_group it should return the socket ID of the first successful socket, while in case of non-blocking mode it always returns 0.

Findings (update):

• #1709 seems to be prematurely merged
• Changes in the refactoring in the group.cpp file could have brought in some of the fixes (counting successful and blocked sockets)

NOT updating, turning to draft.