GridFS: Rename pre- and post-processing funcs

 -  Rename `preProcessingFunc` to `readFilter` and `postProcessingFunc` to `writeFilter`
 -  Consolidate duplicate typedefs
 -  Make `srcData` parameter const
 -  Change binary data buffers from void* to char* for consistency with rest of driver
Merge pull request #110 from noa--/gridfs_pre_and_post_funcs

GridFS: Rename pre- and post-processing funcs