Replace hrtime2text (common.h) with Couchbase::hrtime2text; convert to std::chrono

engines/ep/src/common.h has a function named hrtime2text() which converts a duration in nanoseconds to a human-readable string.

However, we also have an equivalent function in platform/timeutils.h - hrtime2text which does the same thing.

There is no point in having two equivalent functions - we should replace the ep-engine specific one with the one form platform, and update all uses to the platform one.

Additionally, we should also migrate the function over to use the C++11 <chrono> library for the time unit as this is type-safe and allows easier conversion to other durations.