Introduce macro ASCS_UNPACKER_STRIPPED to control i_unpacker's default value of the stripped property.

Add callback registration support for server and object_pool.
Rename file ascs/ext/socket.h to ascs/ext/callbacks.h

Fix ssl graceful shutdown monitoring.

Fix -- in extreme circumstances, messages can leave behind in the send buffer until the next message sending.
Fix -- basic_buffer doesn't support fixed arrays (just in the constructor) in GCC and Clang.
Introduce macro ASCS_CAN_EMPTY_NOT_SAFE for containers used in send/recv buffer whose empty function is not thread safe.

Add downgrade (to normal UDP) test for reliable UDP in udp_test.

For recv_msg, go back to dispatch_strand if macro ASCS_PASSIVE_RECV is not defined because it
 cannot be invoked concurrently without the macro.

More guarantee on object reusing.
A small refactoring.

Fix -- in Windows, a TCP client must explicitly specify a full IP address (not only the port) to connect to.