Allow std::atomic<MonoTime> usage with clang and libstdc++

Summary:
clang couldn't compile libstdc++ atomic<T> unless T is nothrow default constructible.
More details:
- https://stackoverflow.com/a/29483269/461529
- https://bugs.llvm.org/show_bug.cgi?id=33550
- http://www.open-std.org/jtc1/sc22/wg21/docs/cwg_defects.html#1778
Solution for our specific issue is to define MonoTime default ctor as noexcept.

Test Plan: Run `ybd release --sj --clang` on CentOS

Reviewers: mikhail, hector, sergei

Reviewed By: hector, sergei

Subscribers: ybase

Differential Revision: https://phabricator.dev.yugabyte.com/D4298

src/yb/util/monotime.h

@@ -157,7 +157,7 @@ class MonoTime {
   // Return the earliest (minimum) of the two monotimes.
   static const MonoTime& Earliest(const MonoTime& a, const MonoTime& b);
 
-  MonoTime() {}
+  MonoTime() noexcept {}
   MonoTime(std::chrono::steady_clock::time_point value) : value_(value) {} // NOLINT
 
   bool Initialized() const { return value_ != std::chrono::steady_clock::time_point(); }