speed up lookup of JCas cover object from cache

One of the most frequent operations in UIMA user code is to iterate over all FSs of particular type (including subtypes) and do some operation. When using the JCas, the iteration's get() or next() (if JCas Caching is enabled, which is the default) will get the cached JCas cover object corresponding to that Feature Structure and return it.

Look at speeding up this operation, since it's frequent.