Rework API to use just one struct

[nyurik]

Fixes #17

• Now all mock socket instantiations are done with a specific object:

// protocol-specific functions
ServerMocker::tcp()
ServerMocker::tcp_with_port(port)
ServerMocker::udp()
ServerMocker::udp_with_port(port)
// generic function taking anything that
// implements the needed (non-public) trait
ServerMocker::new_with_opts(options_object)

• Fix timeouts - by accident they had 100s instead of 100ms
• Improve PostgreSQL tests with explicit configurations + timeouts + explicit communication stop (to prevent long waits when the client was shutting down)

[nyurik]

One big question I keep having is the better naming... Note that even Clippy complains about the repeated name (you have this lint disabled)... And since renaming crate is not that convenient, some ideas here:

• Server - this is actually a good choice similar to Mockito's mockito::Server -- the idea here is that namespace is technically part of the full name, so in our case the full name is socket_server_mocker::Server (still a duplicate though)
• ServerMocker - redundant with the namespace, but more descriptive if used by itself. This is my top choice.
• SocketMocker - same, and this is my 2nd top choice
• ServerSocketMocker -- this one feels too long and redundant
• Mocker - meh

There are 2 hard problems in computer science: cache invalidation, naming things, and off-by-1 errors.