vlib: increase the stats epoch only when necessary

vlib: increase the stats epoch only when necessary

When the counter vectors are validated and they are already long enough
to fit the given index in memory, there is no need to increase the stats
segment epoch. In this case, the counter vectors do not change as a
result of the validation.

This optimization is necessary for the case when the configuration is
changed at multiple thousands per second rate. The counter vectors grow
at the beginning and their size stabilizes after a while. Without this
improvement, it can still take several seconds for a stats reader to
succeed.

Type: improvement
Signed-off-by: Miklos Tirpak <miklos.tirpak@gmail.com>
Change-Id: I5a6c30255832716a1460018d0bd0f63031de102b