src/utils_cache.c: Very minor performance improvement.

cache_compare() is one of the hottest functions in collectd. Depending how many
metrics collectd handles (how deep the tree is) it gets called an order of
magnitude or two more often than the number of metrics handled. With 100k
metrics, about 6.6% of the CPU time were spent within cache_compare(), with
this commit this reduces to 6.3% (that's, uhm, 4.7% faster?).
src/common.[ch]: Rewrite the escape_slashes() function.

Cleaner and slightly more efficient (3.8% faster) version of escape_slashes().
This function is quite hot with, depending on the configuration, between 1.2%
and 2.6% of total CPU time spent here.