XADD q can lazy allocate pooled chunks

[franz1981]

Xadd qs could lazy allocate the pooled chunk, on demand, instead of eagerly allocate them.
This could be implemented using a plain int field to count the already pooled chunks: this plain int field could be used while appending new chunks, given that is a single writer code.

The interesting thing is that it could save all the configured allocations to ever happen, depending on the load.

In addition it could allow to reuse the max number of allocated chunks (under the max load) by targeting an unbounded number of pooled chunks.