Make Tachyon WriteType configurable.

Tachyon supports kinds of WriteType, like CACHE_THROUGH, MUST_CACHE, TRY_CACHE, and etc. And currently Shark only supports CACHE_THROUGH. Here we make the write type of TachyonOffheapTableWriter configurable, so that the end user can choose different types by "set shark.tachyon.writetype=xxx", to avoid some WRITETHROUGH overhead somehow.