refactor: fixes for futures

[bartlomieju]

This PR is a followup to #3358
After landing it benchmarks exploded indicating problems with workers and deno_core_http_bench.

The bench is already fixed and workers partially.

EDIT: This PR dramatically fixes churn in thread/syscall count that showed up on benchmarks. Thread count is not back to previous levels but difference went from hundreds/thousands to about ~50.

[bartlomieju]

deno/cli/ops/workers.rs

Lines 161 to 173 in 2b3afda

	// Has provided source code, execute immediately.
	if has_source_code {
	js_check(worker.execute(&source_code));
	return Ok(JsonOp::Sync(exec_cb(worker)));
	}
	let op = worker
	.execute_mod_async(&module_specifier, None, false)
	.and_then(move |()| futures::future::ok(exec_cb(worker)));
	let result = tokio_util::block_on(op.boxed())?;
	Ok(JsonOp::Sync(result))
	}

It should use futures::executor::block_on instead of tokio_util::block_on but I'm getting this error:

thread 'tokio-runtime-worker-0' panicked at 'cannot execute `LocalPool` executor from within another executor: EnterError', src/libcore/result.rs:1165:5
stack backtrace:
   0: <std::sys_common::backtrace::_print::DisplayBacktrace as core::fmt::Display>::fmt
   1: core::fmt::write
   2: std::io::Write::write_fmt
   3: std::panicking::default_hook::{{closure}}
   4: std::panicking::default_hook
   5: std::panicking::rust_panic_with_hook
   6: std::panicking::continue_panic_fmt
   7: rust_begin_unwind
   8: core::panicking::panic_fmt
   9: core::result::unwrap_failed
  10: core::result::Result<T,E>::expect
  11: futures_executor::local_pool::run_executor
  12: futures_executor::local_pool::block_on
  13: deno_cli::ops::workers::op_worker_post_message
  14: core::ops::function::Fn::call
  15: deno_cli::state::ThreadSafeState::stateful_op::{{closure}}
  16: deno_cli::ops::dispatch_json::json_op::{{closure}}::{{closure}}
  17: core::result::Result<T,E>::and_then
  18: deno_cli::ops::dispatch_json::json_op::{{closure}}
  19: deno_cli::state::ThreadSafeState::core_op::{{closure}}
  20: <alloc::boxed::Box<F> as core::ops::function::Fn<A>>::call
  21: deno::ops::OpRegistry::call
  22: deno::isolate::Isolate::pre_dispatch
  23: _ZN4deno4SendERKN2v820FunctionCallbackInfoINS0_5ValueEEE
  24: _ZN2v88internal25FunctionCallbackArguments4CallENS0_15CallHandlerInfoE
  25: _ZN2v88internal12_GLOBAL__N_119HandleApiCallHelperILb0EEENS0_11MaybeHandleINS0_6ObjectEEEPNS0_7IsolateENS0_6HandleINS0_10HeapObjectEEESA_NS8_INS0_20FunctionTemplateInfoEEENS8_IS4_EENS0_16BuiltinArgumentsE
  26: _ZN2v88internalL26Builtin_Impl_HandleApiCallENS0_16BuiltinArgumentsEPNS0_7IsolateE
  27: _ZN2v88internal21Builtin_HandleApiCallEiPmPNS0_7IsolateE
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.
fatal runtime error: failed to initiate panic, error 5
Abort trap: 6

[ry]

LGTM - thanks - I agree we just need to iterate on this a bit.