Add support for high volume package structures

[datadavev]

There is a need to record arbitrary folder structures to represent the arrangement of files within a data package.

The approach taken here is to record the folder structure using an sqlite database file, with one db file per package. The db will be stored as an object in hash store. An additional db will be used to provide a reverse index of object identifier to package.

[datadavev]

Initial proof of concept implementation in PR #153. The approach needs to be evaluated for impact on upstream components since it includes a significant change in the behavior of PIDs where an alternate form is introduced to include an optional path component appended to a PID.

The implementation supports reuse of existing folder elements after mutation with only the subfolder containing a change and it's parents are updated. Siblings remain unchanged.