Avoid allocating functions and use at most one timer per interval in throttleLeadingAndTrailing

[mrxz]

Description:
The implementation of throttleLeadingAndTrailing was very inefficient when many updates took place in short succession. Virtually all invocations would result in clearing a timer, allocating a new function and creating a new timer. This impacted both the performance and memory usage of setAttribute.

Using the performance/set-attribute and performance/animation-set-attribute benchmarks showed a clear performance benefit (when congested) cutting the duration in half (~9.16µs/op -> ~4.04µs/op) and reduced memory consumption:

Instead of allocating a new function for each timer, one is created in advance. In case calls take place within the minimumInterval only one timer is created and re-used until it expires ensuring at most one timer per interval (if needed).

Changes proposed:

• Pre-allocate timeout callback function
• Avoid clearing and recreating timeout and instead create and re-use one timeout per interval (if needed)

[dmarcos]

can do a single line

deferTimer = deferTimer || setTimeout(timerExpired, minimumInterval - sinceLastTime);

[mrxz]

Done

[dmarcos]

Thanks!