Change client_t.protocol from pointer to buffer

Fix observed crash when shutting down services server where the client_t
protocol was pointing to a freed service's protocol description string.

This was the last pointer in client_t and should resolve the last race
conditions (memory ownership issues) with its data members.

This also resolves a small memory leak in getnodeclient() where the last
client "gotten" would have its heap-duplicated protocol string leaked.