Additionally, instead of polling ops in a loop until none of them are
ready, the isolate will now yield to the task system after delivering
the first batch of completed ops to the javascript side.
Although this makes performance a bit worse (about 15% fewer
requests/second on the 'deno_core_http_bench' benchmark), we feel that
the advantages are worth it:
* It resolves the extremely high worst-case latency that we were seeing
on deno_core_http_bench, in particular when using the multi-threaded
Tokio runtime, which would sometimes exceed a full second.
* Before this patch, the implementation of Isolate::poll() had to loop
through all sub-futures and poll each one of them, which doesn't scale
well as the number of futures managed by the isolate goes up. This
could lead to poor performance when e.g. a server is servicing
thousands of connected clients.
This change is made in preparation for using FuturesUnordered to track
futures that are spawned by the isolate. FuturesUnordered sets up
notififications for every future that it finds to be not ready when
polled, which causes a crash if attempted outside of a task context.
* Moves how snapshots are supplied to the Isolate. Previously they were
given by Behavior::startup_data() but it was only called once at
startup. It makes more sense (and simplifies Behavior) to pass it to the
constructor of Isolate.
* Adds new libdeno type deno_snapshot instead of overloading
deno_buf.
* Adds new libdeno method to delete snapshot deno_snapshot_delete().
* Renames deno_get_snapshot() to deno_snapshot_new().
* Makes StartupData hold references to snapshots. This was implicit when
it previously held a deno_buf but is made explicit now. Note that
include_bytes!() returns a &'static [u8] and we want to avoid
copying that.
