reader_permit: reader_resources: add operator- and operator+
reader_permit: reader_resources: add operator- and operator+

In addition to the already available operator+= and operator-=.

avikivity/scylla,scylladb/scylla,avikivity/scylla,scylladb/scylla,scylladb/scylla,scylladb/scylla,avikivity/scylla