Create connection from socket fd

[locker]

Currently, to create a connection, the user has to specify a URI. The connector creates a socket and connects it to the given URI under the hood. There are use cases when the user doesn't have a URI but does have a connected socket fd. For example, a socket fd could be inherited from the parent process on fork. We need a way to wrap a raw socket fd number in a connection object.

For more details see the IPROTO over socketpair design document.