Applies micro-optimizations and removes obsolete settings

[sorvell]

Applies micro-optimizations that were found to improve element creation benchmarks by 5-10%, and removes obsolete settings:

• Removed legacyNoBatch and legacyNotifyOrder settings.
• dom-if/repeat: dom-change and renderedCount no longer fire with legacyOptimizations set.
• LegacyElementMixin: isAttached now set before calling connectedCallback so it is batched with initial rendering.
• PropertiesChanged: property accessor code now inlined for efficiency rather than calling _get/_setProperty. The __dataCounter tracking flag has been moved here to avoid the need to override _flushProperties in PropertyEffects.
• PropertyEffects: inlined runEffectsForProperty into runEffects for efficiency. Removed wrapping around sending data events.
• async: In the microtask scheduler, now only provoke a DOM mutation if needed.

[kevinpschaaf]

Based on in-the-wild code relying on isAttached to be set after all observers have run, probably need to take out the isAttached change.