Provenance store

[balis]

• Set up a provenance store (database) suitable for the HyperFlow provenance model
• Implement "flushing" provenance events logged during workflow execution to this store
• The store needs to support common provenance queries (such as lineage graph of a data/signal)
• Probably it should be a more general store for "workflow execution history" which supports not only provenance queries but also others

Provenance logging is implemented in engine2/process.js
By default it is disabled by the flag set in engine2/index.js:
this.logProvenance = false;

[mpawlik]

First prototype is ready, provenance data, in form of event nodes (tuples) is stored in neo4j database, with two relationship types:

• events connected by signal (signal index) write/read
• signal writes dependant on other signal reads

TODOs:

• fix lack of optimization, and suspiciously slow writes to db
• add support for simultaneous execution of multiple workflows
• fix lots of callback nesting
• think about special treatment of signal events with initial values (maybe 'virtual write' events?)