[worker] Introduce new config key identity_suffix

May now distinguish worker activity on different hosts with identical names.

Compatibility note: the non-default identity still conforms to

hostname, pid = identity.split(":")
[spec] detect colliding method definitions
[worker] Cannot use abstract methods in worker.rb

Methods would be unexpectedly redefined when Modules are combined in Ntswf::Instance
[spec] Separated method collision spec
[worker] Share options, not polling

Maintainer feels better this way.
Merge pull request #18 from phorsuedzie/identity-suffix

Identity suffix