Perf: `DynamicCapacity.size()` calls megamorphic `RetainableByteBuffer.remaining()`

**Jetty version(s)**
12.1.x

**Description**
A H2 benchmark revealed that the H2 `Generator` often calls `DynamicCapacity.ensure()` (via the different `put*()` methods) which itself calls `size()` which eventually calls `RetainableByteBuffer.remaining()` method in a loop.

Async-profiler revealed that `RetainableByteBuffer.remaining()` is megamorphic, so calling it is very expensive and that is done very liberally as the assumption is that this call would be cheap.

This should be improved, maybe by tracking the `RetainableByteBuffer` remaining bytes in another variable instead of summing them up each time `size()` is called?