Use monotonic_time() to compute elapsed time

... not `erlang:now()` (deprecated) or `os:timestamp()` (which could go
backward).

file_handle_cache:
    While here, change the LRU tree key from `erlang:now()` to a
    combination of the monotonic time and the handle reference (which
    was the value in the tree previously). This is required as the
    monotonic time doesn't increase strictly: two subsequent calls
    could return the same time, leading to non-unique key. We could use
    unique_integer([monotonic]) but it's a bit heavy weight just to keep
    a rough order of opened files. So now, files sharing the same age
    are arbitrary "sorted" by their reference.

References #233.

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