perf: avoid multiple calls to runMicrotask

[littledivy]

Improves HTTP throughput by 8-9k rps on Linux:

this patch

Requests/sec: 145001.69
Transfer/sec:     20.74MB

main

Requests/sec: 137866.61
Transfer/sec:     19.72MB

The improvements comes from the reduced number of calls to op_run_microtask per request. Returning true from a macrotask callback already calls op_run_microtask so the extra call was redundant.

Here's --strace-ops output for a single request:

main

[     4.667] op_http_wait                                       : CompletedAsync Async
[     4.667] op_run_microtasks                                  : Dispatched Slow
[     4.668] op_http_try_wait                                   : Dispatched Slow
[     4.668] op_http_try_wait                                   : Completed Slow
[     4.668] op_http_wait                                       : Dispatched Async
[     4.668] op_http_set_response_header                        : Dispatched Slow
[     4.668] op_http_set_response_header                        : Completed Slow
[     4.669] op_http_set_response_body_text                     : Dispatched Slow
[     4.669] op_http_set_response_body_text                     : Completed Slow
[     4.669] op_run_microtasks                                  : Completed Slow
[     4.669] op_has_tick_scheduled                              : Dispatched Slow
[     4.669] op_has_tick_scheduled                              : Completed Slow
[     4.669] op_run_microtasks                                  : Dispatched Slow
[     4.669] op_run_microtasks                                  : Completed Slow
[     4.669] op_run_microtasks                                  : Dispatched Slow
[     4.669] op_run_microtasks                                  : Completed Slow

this pr

[     3.726] op_http_wait                                       : CompletedAsync Async
[     3.727] op_run_microtasks                                  : Dispatched Slow
[     3.727] op_http_try_wait                                   : Dispatched Slow
[     3.727] op_http_try_wait                                   : Completed Slow
[     3.727] op_http_wait                                       : Dispatched Async
[     3.727] op_http_set_response_header                        : Dispatched Slow
[     3.728] op_http_set_response_header                        : Completed Slow
[     3.728] op_http_set_response_body_text                     : Dispatched Slow
[     3.728] op_http_set_response_body_text                     : Completed Slow
[     3.728] op_run_microtasks                                  : Completed Slow
[     3.728] op_run_microtasks                                  : Dispatched Slow
[     3.728] op_run_microtasks                                  : Completed Slow