unix: use uv__hrtime() internally

This commit renames the various uv_hrtime() implementations to uv__hrtime().

Libuv uses the high-res timer internally in performance-critical code paths.
Calling the non-public version avoids going through the PLT when libuv is
compiled as a shared object.

The exported uv_hrtime() now has a single definition in src/unix/core.c that
calls uv__hrtime().

A future optimization is to lift the uv__hrtime() declarations into header
files so they can be inlined at the call sites. Then again, linking with -flto
should accomplish the same thing.