folly: avoid new warnings from -Winconsistent-missing-override

folly: avoid new warnings from -Winconsistent-missing-override

Summary:
Upgrading to clang:dev (clang >3.6) brought in some new warnings.
This change address all of the issues exposed by the new
-Winconsistent-missing-override, usually by simply adding the
missing "override" keyword.  However, in
folly/wangle/channel/test/MockChannelHandler.h, I chose
to ignore those warnings for the mocked functions.
* folly/futures/test/ViaTest.cpp: Add missing "override"(s).
* folly/io/async/AsyncSSLSocket.h: Likewise.
* folly/io/async/AsyncSocket.h: Likewise.
* folly/io/async/EventBase.h: Likewise.
* folly/test/ExceptionWrapperTest.cpp: Likewise.
* folly/wangle/channel/AsyncSocketHandler.h: Likewise.
* folly/wangle/channel/test/MockChannelHandler.h: Ignore
the new warning for these functions.

Test Plan:
Run these commands and note there are fewer errors than before:
fbconfig --clang --with-project-version=clang:dev -r folly && fbmake dbgo

Reviewed By: hans@fb.com

Subscribers: trunkagent, fugalh, folly-diffs@, jsedgwick, yfeldblum

FB internal diff: D1848330

Tasks: 6244745

Signature: t1:1848330:1423858909:b167ca220d6c9fe036d3adca08cf3053a7a9de16