* swarm/pss: fix data race on HandshakeController.symKeyIndex

The HandshakeController.symKeyIndex map was accessed concurrently.
Since insufficient test coverage the race is not detected every time.
However, running TestClientHandshake a 100 times seems to be enough to
reproduce the race.

Note: I've chosen HandshakeController.lock to protect
HandshakeController.symKeyIndex as that was already protected in a few
functions by that lock.

Additionally:
- removed unused testStore
- enabled tests in handshake_test.go as they pass
- removed code duplication by adding getSymKey()

* swarm/pss: fix a data race on HandshakeController.keyC

* swarm/pss: fix data races with on Pss.symKeyPool