Feature/boost datetime implementation

[conlain-k]

Addresses #1523.

This PR is ready for review.

Risk

This PR makes major API changes.

Testing Plan

Use ATF and unit tests to ensure that code using datetime utilities still works correctly

Summary

Remove uses of struct timeval and replace it with boost::posix_time::time_duration, allowing platform-independent usage and cleaner use/helper functions. Boost handles most helper functions, with topical wrappers to preserve old functionality and abstract from the underlying implementation.
Also removes useless date_time::DateTime class that just held static members in favor of a flat date_time namespace containing all helper functions.

Note that src/components/telemetry_monitor/src/telemetry_monitor.cc still uses struct timeval as required for the select POSIX call; this should eventually be replaced as well.

Tasks Remaining:

• Testing for changed behavior -- use of old struct timeval allowed some poorly-defined behavior such as overflowing the tv_usec field (over 1 million microseconds). Boost's implementation doesn't allow mutating specific fields, meaning that changing said behavior could cause a bug.

CLA

• I have signed the CLA

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