Have EventBase implement wangle::Executor

Have EventBase implement wangle::Executor

Summary:
It already does the work (`runInEventBaseThread`) but it will now be convenient to pass an `EventBase` where wangle wants an `Executor`.

Had to rip off the `boost::noncopyable` from `wangle::Executor` which is an interface and does not require non-copyability so that didn't really belong there in the first place I think. (Without this change, you get an obscure compiler error because of the double-inheritance from `boost::noncopyable`).

Test Plan: Things build, tests pass

Reviewed By: davejwatson@fb.com

Subscribers: jsedgwick, trunkagent, fugalh, exa, njormrod, folly-diffs@, andrii

FB internal diff: D1671500

Signature: t1:1671500:1415727572:a7dba33c669ca122aecaee3c700f9e53e54838d1