pause: when changing pause state, update socket state

Especially unpausing a transfer might have to move the socket back to the
"currently used sockets" hash to get monitored. Otherwise it would never get
any more data and get stuck. Easily triggered with pausing using the
multi_socket API.

Reported-by: Philip Prindeville
Bug: https://curl.haxx.se/mail/lib-2018-03/0048.html
Fixes #2393
Closes #2391