Preallocate the map in Aggregation process

[srikartati]

Describe the bug
In scale setups, memory utilization of aggregation process can be very high.

In PoCs with preallocation, total memory consumption went down by 20-30% but there is no effect in the difference between sys and heap usage; there is still fragmentation.

To Reproduce
We already see this in Antrea performance tests.

Expected behavior
Cut down the memory consumption.

Versions
Please provide the version of go-ipfix where this problem is observed.

[srikartati]

@wsquan171 Please add if I missed something.