Reduce resize in rate aggregation (elastic#136734)

Previously, we tried to reduce buffer resizing by over-allocating memory 
up to the remaining size of the block when buffering data points for the
rate aggregation. However, we hit a circuit breaker in extreme cases
where there are many time series but few data points per series. This
change attempts to identify as many groups as possible and allocate the
required memory to minimize buffer resizing.

Author: dnhatn