Add CellComparator to HFileContext

The HFileContext is present when reading and writing files. It is populated at read time using HFile trailer content and file metadata. At write time, we create it up front.

Interesting is that though CellComparator is written to the HFile trailer, and parse of the Trailer creates an HFileInfo which builds the HFileContext at read time, the HFileContext does not expose what CellComparator to use decoding and seeking. Around the codebase there are various compensations made for this lack with decoders that actually have a decoding context (with a reference to the hfilecontext), hard-coding use of the default CellComparator. StoreFileInfo will use default if not passed a comparator (even though we'd just read the trailer and even though it has reference to filecontext) and HFile does similar. What CellComparator to use in a given context is confused.

Let me fix this situation removing ambiguity. It will also fix bugs in parent issue where UTs are failing because wrong CellComparator is being used.