Code cleanup

Code review: Yannick Lecaillez

DefaultIndex.java:
Added Reject.ifNull() in a few places.
In importPut() and importRemove(), extracted local variable for entryIDSet.
In importRemove(), inverted if statement + improved thrown exception
In several methods used key.toHexPlusAsciiString() instead of StaticUtils.byteArrayToHexPlusAscii().
In computeEntryIDSet(), extracted nbDeleted local variable + simplified the code.