Add the possibility to specify memcached's chunk growth factor.

Memcached's -f command line option specifies the chunk size growth factor,
namely the value used to create slab classes. We use also the -n value set to
5, meaning that slab classes will start from 5 bytes and grow multipling each
time by 1.05 and rounding the value to the first one divisible by 8.

Bug: T129963
Change-Id: I5579ce1ce57615feac1d432845b59488d5e93fec