tstime/mono: remove package

[crawshaw]

The premise of tstime/mono is that time.Now is too slow because it looks up time twice.

@dsnet points out that Russ has an idiom in golang/go#61765 (comment) for using the time package to get efficient monotonic time, that is easier to read than a custom Go package.

First define a single global time:

var globalStart = time.Now()

Then in any hot path, compute monotonic time by looking at the time since globalStart. E.g. to compute a duration:

start := time.Since(globalStart)
// ...
d := time.Since(globalStart) - start

Similarly if you want to log the current mono time it is globalStart.Add(time.Since(globalStart)).

We could replace the unsafe machinery in the mono package with this, but given that it is generally easier to read code that depends on fewer bespoke packages and types, I propose we remove the mono package altogether and adopt this idiom where we need the performance.

[bradfitz]

Yes, please.