Added SA sampling strategy dependent on BWT chars.

Strategy sa_bwt_sampling samples every sa_sample_dens
suffix in text order + all suffixes x=SA[i], with
BWT[i] \in BWT_SAMPLE_CHARS.
BWT_SMAPLE_CHARS is expected to be stored in a
serialized int_vector<> in the cache_config with
key sdsl::constant::KEY_SAMPLE_CHAR.