add monotonic clock "pause" feature to p1->p2 adapter

[dicej]

This adds a new adapter_monotonic_clock_set_paused export to the adapter which toggles whether clock_time_get will call monotonic_clock::now when called with CLOCKID_MONOTONIC versus returning a cached value instead.

This is a workaround for guest language runtimes whose cabi_realloc implementations may call clock_time_get. Since calling imports from cabi_realloc is disallowed, this will trap if the adapter calls monotonic_clock::now. We can avoid the trap by using a cached value instead.

This helps us address
bytecodealliance/componentize-go#56. In that case, cabi_realloc is implemented by calling
unsafe.SliceData to allocate a segment of the appropriate size and alignment from the GC-managed heap and then pinning it using pinner.Pin until it is no longer needed. However, such an allocation may trigger a GC under memory pressure, and as of this writing the Go collector calls clock_time_get to measure time spent in various stages of the GC process.

An alternative fix for the componentize-go issue would be to modify the Go runtime to avoid such calls on WASI during GC, but getting that upstream is likely to be significantly more difficult than working around it in the adapter. If and when Go supports WASIp3 or later natively, the adapter will no longer be used, in which case we will certainly address this upstream.

[alexcrichton]

I'm not thrilled about it, but I suspect you aren't either, and so long as Go is an unchangable monolith I don't think there are many other realistic options on the table.