DS-1205 Adds support for read-only contexts that bypass internal DSO cache

https://jira.duraspace.org/browse/DS-1205

As requested, this pull contains an API improvement to address the issue in DS-1205 - Context caches that can exhaust JVM memory. It introduces a new Context option - READ_ONLY - which is created with a new constructor invocation:  

Context c = new Context(Context.READ_ONLY);

Read-only contexts will not populate the internal object's cache, so will have a constant zero-memory footprint over the life of the context object. Run-time exceptions will be thrown when a read-only context is used to update DSpaceObjects. Read-only contexts may prove to have broad applicability beyond the specific use-case of DS-1205.

Contexts created without the option should behave as before.  Note that this code has not been tested.