OPENDJ-1724 (CR-5789) Persistit: add support for verify-indexes

Replaced "instanceof jeb.BackendImpl" by calling the new Backend.supportsIndexes().
Pulled up verifyBackend() to Backend class + removed the Entry parameter since it is only used to read the errorCount which is already returned by the verifyBackend() method.


Backend.java:
Added supportsIndexes() and copied verifyBackend() signature from jeb.BackendImpl + removed the Entry parameter since it is only used to read the errorCount which is already returned by the verifyBackend() method.

BackendImpl.java:
Overrode supportsIndexes() + @Override verifyBackend().

jeb.VerifyConfig.java: REMOVED superseded by pluggable.VerifyConfig.

VerifyJob.java:
Replaced jeb.VerifyConfig with pluggable.VerifyConfig.
In verifyBackend(), removed now unused Entry parameter.
Removed addStatEntry().

VerifyConfig.java:
Simplified code in constructor.
In add*() method, added parameter validation.
Made getters return List instead of ArrayList.

VerifyIndex.java:
In mainVerifyIndex(), called Backend.supportsIndexes() instead of "instanceof jeb.BackendImpl".

TestImportJob.java, TestRebuildJob.java, TestVerifyJob.java:
Consequence of pulling verifyBackend() to Backend:
- removed casts to jeb.BackendImpl.
- read errorCount from verifyBackend() result value.
- removed errorCount constant fields.
- renamed be and beID fields to backend and and backendID .
- removed bldStatEntry() and getStatEntryCount() method now unused.