fix addr leak in create_write_persist ult

Closes #20

See merge request !12

- fixes #20

Refactoring to enable generic backend interface

See merge request !11

- we can retrieve from git history if needed; in it's current form the
  code is out of date with current bake API

- use bulk poolset in place of bulk pools for more size options
- allow pipeline/memcpy on smaller transfers
- no explicit limit on courrency; one ult per chunk
- work towards generalizing configuration parameter api
- automatic defaults for buffer settings if pipelining enabled

- combine data transfer portions of write and create_write_persist into
  one subroutine

Using ULTs to do transfers in parallel

See merge request !9