LoopingCall should parameterize its scheduler

[radix]

_callLater and _seconds should both be provided or obsoleted by parameterized functions or a Clock/reactor-like object.

[radix]

Up for review at parameterize-loopingcall-scheduler-2633, which also attempts to resolve #2634.

[therve]

• I don't really like the setClock method. I mean, anybody can do loop.clock = foo, right?
• the variable new_time in t.i.base should be named newTime
• the docstring of seconds in t.i.interfaces should be

  """
  blah.
  """
  

  Don't hesitate to clean the docstrings of IReactorTime :)
• the test_seconds is a bit insane, but I guess you have a reason for that

Apart from nitpicking, that looks really great, so please merge when you're ok.

[therve]

One last thing: can you update the copyright notices of the touched files? Thanks.

[radix]

(In [20273]) Merge parameterize-looping-call-2633.

Author: radix Reviewer: therve Fixes #2633 Fixes #2634

LoopingCall (and code which uses it) is now much more testable because the scheduler it uses is overridable. In support of this, a 'seconds' method was added to the reactor (IReactorTime) to strengthen the logical grouping of 'what is the current scheduling-time' and 'schedule a call for later'.