RGW: make SSTR macro safe against variable name collisions

RGW: make SSTR macro safe against variable name collisions

The original SSTR macro defined a local variable `std::stringstream ss`
inside its expansion. When an expression that used a variable named `ss`
is passed to this macro, it led to name shadowing and unexpected results.

  Example:
    std::stringstream ss;
    ss << "aaa";
    auto result = SSTR("this is ss=" << ss.str());

  Actual result:
    "this is ss=this is ss="
  Expected result:
    "this is ss=aaa"

This change rewrites the macro to construct an unnamed temporary to
avoid any name collision.

Signed-off-by: Oguzhan Ozmen <oozmen@bloomberg.net>