0.17.1 release keyspace support

* Add new error type for when a collection no longer exists

* Add leaky initial implementation of drop_tree. Cut new versions

* Use explicit EBR flush and drop in drop_tree test

* Flush EBR more regularly in removal test

Release 0.16

Merge pull request spacejam#343 from spacejam/tyler_2018_prep

2018 prep

Bump dependencies up, slightly refactor some lock-free code

Merge pull request spacejam#333 from spacejam/tyler_blob_storage

support for off-log external blobs. closes spacejam#198.