Adding IO stats in AsyncIO.

Summary:
It would be great if we have IO stats tracking built within AsyncIO.
It would enable upper layer application to better track the number of I/O
that was submitted and completed

Test Plan:
$ fbmake runtests
Test Results Summary:
Passed: 1734
100% successful

Reviewed By: philipp@fb.com

Subscribers: njormrod, schen, stanislav

FB internal diff: D1557472

Tasks: 5008299
make with_exception() const work in all cases

Summary:
Abstract the with_exception() logic into a static template
function which will work with a const or non-const object.

Test Plan: exception_wrapper_test

Reviewed By: vloh@fb.com

Subscribers: njormrod

FB internal diff: D1549175

Tasks: 5127267
Fix folly signed/unsigned comparisons

Summary:
Fix a few sign comparison issues in folly::IPAddress

@override-unit-failures

Test Plan: existing tests, compiled with clang

Reviewed By: meyering@fb.com, davejwatson@fb.com

Subscribers: njormrod, bmatheny, subodh, ranjeeth, pgriess

FB internal diff: D1559152
ThreadPoolExecutor and its children CPUThreadPoolExecutor and IOThreadPoolExecutor

Summary:
Spun off from https://phabricator.fb.com/D1534506 as this seemed different enough for a new diff

Similar to previous diff but attempts to reuse a common thread management process between cpu and io bound thread pools. Also sets the stage for other common functionality, e.g. stats, monitoring, timeouts, and so on

Here is some output from the queue benchmark in common/concurrent with both of these pools added (changes to BM not in this diff): https://phabricator.fb.com/P16308560

Test Plan: added a unit test, ran benchmark

Reviewed By: davejwatson@fb.com

Subscribers: fugalh, njormrod, bmatheny

FB internal diff: D1555443

Tasks: 5002392, 5002425
move connectionmanager/managed connection to wangle

Summary: Moar code sharing between proxygen/thrift.  Code must be moved to a common library: The only one we currently have is folly, but if someone wants to argue for a separate lib, that's cool too

Test Plan: fbconfig -r proxygen; fbmake dev

Reviewed By: dcsommer@fb.com

Subscribers: doug, fugalh, bmatheny, njormrod

FB internal diff: D1519919

Tasks: 5002343
Bump version to 7:0