Add allocator constructors for strand

[chriskohlhoff]

[chriskohlhoff]

Pre-Lenexa Summary

[async.strand]

In Cologne it was requested that the strand<> template provide allocator constructors. These allocators would be used to allocate any internal data structures required by the strand (such as a queue for the pending function objects).

I suggest that an allocator-aware constructor be added as a counterpart only for the following constructor:

explicit strand(Executor ex);

i.e.:

template <class Allocator>
strand(Executor ex, const Allocator& a);