[vm] Buffer writes in TimelineEventFileRecorderBase

Previous implementation allocated individual events and then
executed writes directly - both of these add unnecessary overhead.

Instead allocate and process events in blocks just like other
recorders and buffer writes in a temporary buffer before passing
data to Dart::file_write_callback().

Currently if you write something like this:

    int fib(int n) {
      return Timeline.timeSync('fib', () {
        // typical recursive fib implementation here.
      });
    }

    for (var i = 0; i < 1000; i++) {
      fib(10);
    }

and then run it with perfettofile recorder you end up waiting several
seconds before recorder is drained with program spending most time
doing small writes via File::WriteFully.

This is no longer the case after this change.

TEST=ci

Change-Id: I6a33ccdb1f293a94669ad0dd7d883837b86c8b6f
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/426861
Commit-Queue: Slava Egorov <vegorov@google.com>
Reviewed-by: Ryan Macnak <rmacnak@google.com>