MINOR: stats: support rate in stats-file

MINOR: stats: support rate in stats-file

Implement support for FN_RATE stat column into stat-file.

For the output part, only minimal change is required. Reuse the function
read_freq_ctr() to print the same value in both stats output and
stats-file dump.

For counter preloading, define a new utility function
preload_freq_ctr(). This can be used to initialize a freq-ctr type by
preloading previous period value. Reuse this function in load_ctr()
during stats-file parsing.

At the moment, no rate column is defined as generic. Thus, this commit
does not have functional change. This will be changed as soon as FN_RATE
are converted to generic columns.