* split out erasure into new crate; add implementation using rust reed-solomon-library
* Track erasures with a &[bool] instead of indexes
* fix bug that reported the number of erasures incorrectly
* Introduce erasure `Session` for consistent config
* Increase test coverage; fix bugs
* Add ability to remove blobs from erasure meta tracking. test added
* Track deletion of coding blobs in blocktree via ErasureMeta. Added to
test
* Remove unused functions in blocktree
* add randomness to recovery thread to exercise recovery due to either new
data or coding blobs
* Add unit test for ErasureMeta index handling
* Re-enable test in broadcast stage
