Make TimeoutId an interface to allow custom implementations

[leozilla]

The TimeoutId type could be an interface so that a TimeoutManager implementation can choose a suitable implementation.
Implementations could be UUTimeoutId, StringTimeoutId and NumericTimeoutId.

When storing the timeout state in a (distributed) database you could use the db key as the TimeoutId.

[Domo42]

already done with the lastest commit 0902b5a in the master branch

[Domo42]

implemented in master branch