DRILL-5319: Refactor "contexts" for unit testing closes #787
This PR is purely a refactoring: no functionality is added or changed.
The refactoring splits various context and related classes into a set
of new interfaces with needed for operator-level unit tests. The other,
Drillbit-related methods are left in the original interfaces. Most code
need not change.
The changes here allow operator-level unit tests to mock up the
exec-time methods so we can use them without firing up a Drillbit (or
using mocking libraries).
A later PR will provide the sub-operator test framework that uses this
refactoring.
Changes include:
* The OptionManager is split, with read-only methods moving to a new
OptionSet interface.
* The FragmentContext is split, with an exec-only FragmentExecContext
proving low-level methods.
* OperatorStats is split, with a new OperatorStatReceiver class
providing write-only support to operators.
* Several places that accepted an OperatorContext or FragmentContext,
but needed only an allocator, are changed to accept the allocator
directly.
Includes fixes for code review comments
Adds more comments. Postpones the suggested rename until all affected
code is in master, else it will be difficult to synchronize the rename
across multiple branches.
