core: time arithmetic improvements

This patch addresses issue bugzilla:1900. It specifically addresses the comment in the issue regarding missing arithmetic operators. Providing a more complete set of arithmetic operators will help reduce the number of conversions that need to be done on Time.

Improve time-arithmetic consistency and code duplication by replacing the multiple operand overloaded methods with a template for standard integer types.

We have additional patches to the other modules that use the new arithmetic operations to remove explicit conversions of Time to double and back.