Make threading macros follow C syntax

[yuyichao]

So that it doesn't confuse people reading the code or mess with editor formatting.

1. Require semicolon after JL_(UN)LOCK.

2. Make FOR_*_HEAP macros (use and) behave like for loops.

   • The restructuring of some FOR_CURRENT_HEAP is because GCC and Clang cannot figure out (before optimization) that the loop body (with a single return) is always executed and give a warning about not returning a value when reaching the end of the function.
   • The FOR_*_HEAP macros are changed to function form to match C control flow syntax and possibly help editors to understand that it's not a type declaration. It doesn't really affect formatting and can be removed if people hate it.....

   @tkelman Someone claimed that the for loop trick might not work with MSVC. I guess it's due to declaration of an anonymous struct in the for loop or because MSVC doesn't support this C99 feature in C mode(edit: not for this reason, the example in that thread was also C++). I think this patch should be fine since the struct is declared outside the loop and we are compiling with C++. Could you please verify?

[yuyichao]

= = .....

• AppVeyor Win32 is the intermittent SmallVector failure....
• Travis/Push lin32 has a network issue....
• Travis/Push lin64 is a pollfd failure that I don't remember seeing before (although I remember there was a known problem with the pollfd test that I didn't really follow) (pollfd test fails for small intervals #12824)

Force push to trigger another build to see if any of these issues presist.

[tkelman]

Looks like MSVC 2013 handles this fine, though I haven't attempted building the threading code under MSVC.

[yuyichao]

The syntax I'm worrying about is in both threading and non-threading version of the code.

[yuyichao]

I added a note about the gcc/clang warning bug. Will merge tomorrow if people are fine with this...