socket leak

mod-notify leaks a socket when performing POST /notify ..

Can be demonstrated in a unit test by repeated calls to POST /notify.

It is also clear from code inspection that a WebClient is constructed for calls to postNotify -> getOkapiModulesClient - > OkapiModulesClientImpl.

OkapiModulesClientImpl never closes webClient (not that it should, but better than creating a pool every time)

Instead OkapiModulesClientImpl should create only one WebClient per Vert.x instance.

Repeating test SendNotifyTest.testValidRequest is a way to reveal that number of sockets. Unfortunately, not so easy to
make a test for number of sockets in use.