Enabling folly::Synchronized for non-default constructible objects

Enabling folly::Synchronized for non-default constructible objects

Summary:
Currently folly::Synchronized can't be used with non-default
constructible types because copy and move constructors require
creation of datum_ before it's assignment.
To fix this we can initialize them in initializer-list and
handle guard creation using helper constructor

Test Plan:
fbconfig -r folly && fbmake runtests
Fixes compilation error: Synchronized started working for non-default
constructible types in my project(rfe).

Reviewed By: yuri@fb.com

Subscribers: dhruvbird

FB internal diff: D1502110