EPIC: Model update/load race conditions handling

[dlamoris]

Problem

For a given branch, SPARQL update and GSP endpoints can be used to update the rdf model. If 2 or more of these actions happen at the same time, complex race conditions can occur.

Proposal

A simpler way to prevent this is to just enforce one update at a time for a given branch - if a transaction is already underway for a branch, reject the update/load request with 409. This allows the underlying logic to be optimized/simplified without needing to consider race conditions. Currently transaction info is written to the triplestore and deleted after transaction is complete - can check for existence of said transaction for a given branch when a request comes in.

Other considerations

Need to determine if branch/lock creation needs to be included/granularity (creating a branch from a branch currently under transaction vs unrelated branch)