Support vlen r/w with simple parent types

[mattjala]

There aren't currently any tests in the API tests/vol-tests that read and write with variable length sequences, so this needs more comprehensive testing before it's ready to go in. Point and hyperslab selection also still need to be implemented

[mattjala]

• Made selection compatible with type conversion in RV_dataset_write by performing H5Dgather before H5Tconvert when the selection is noncontiguous.
• Refactored the write pipeline to use a unique buffer for each step that uses dynamic allocation: gather_buf for gathering selections, selection_buf for putting information about point selections into the body, base64_encoded_values for the encoded values during point selection,tconv_buf/bkg_buf for type converison, and serialize_buf for serialization to json/binary. uinfo.buffer is updated after each step to always points at the buffer containing the processed data for the write.

[mattjala]

• Variable length I/O is now done through binary rather than JSON for non-point selections
• HSDS doesn't accept point selections as a request parameters, so point selection is still done through JSON
• Variable length point selection depends on HDFGroup/hsds#325