Improve performance of graph operations.

Three separate sources of slowness have been identified:

1. Early in the process when data is being gathered, profiling shows Hibernate eating a lot of memory. Caches may need configuration and later versions of Hibernate may make that easier. GraphTraversal will tend not to be querying the same objects repeatedly.

2. In reviewing the data gathered via Hibernate, GraphPolicyRule eats much CPU time in rule-matching. The internal implementation of the class could do non-trivial static analysis when initialized, compiling the list of rules to a decision tree of checks, branches, changes, making the subsequent matching much faster.

3. In actioning the model changes, PostgreSQL can spend much time in managing the _reindexing_required table. Perhaps migration to https://github.com/glencoesoftware/omero-es can somehow help.

Start by profiling the three named areas: Hibernate during data gathering, GraphPolicyRule during rule matching, and PostgreSQL work involving the _reindexing_required table. Establish separate benchmarks and narrow the issue to a concrete bottleneck before changing implementation; done should mean a measured performance improvement in the affected graph operations.