MB-15687 index_entry: Fix incorrect key prefix comparison
MB-15687 index_entry: Fix incorrect key prefix comparison

When then encoded length of user supplied key is greater than
length of encoded index entry, prefix comparator will always return 1.
Fixed prefix comparator to determine index entry's keylen and use that
as prefix length for comparator.

Change-Id: If4605761b01ac170db0c60b3ccc04515c24c8358

jchris/indexing,jchris/indexing,jchris/indexing,jchris/indexing