CPP-999 Compilation error in cpp-driver-2.16.2 when using C++98 standard

[absurdfarce]

Remove C++11 usage in favor of a struct constructor to be explicit about the zero value init

[absurdfarce]

Don't overthink this; let's solve this the easy way via a base constructor for the snapshot struct. Makes sense for that as a default anyway and you avoid any concerns about default val initialization.

[absurdfarce]

Things to look into/fix after review with @lukasz-antoniak:

• Add the explicit call to zero arg constructor for zero_snapshot_ (just to be explicit)
• Can we create a singleton zero_snapshot_ rather than a per-histogram val?