We cannot skip major Lucene version and upgrade directly from 5 to 7,
because backwards codec compatibility layer only supports one major
release. To support online reindexing we have to do the upgrade to
Lucene 7 in two steps and wait for one gerrit release between the
Lucene version upgrades.
There are some removal and deprecation compared to current 5.5 version:
* IntField, LongField are deprecated and supposed to be replaced with
IntPoint and LongPoint. We use the deprecated Legacy<Foo> field types.
These types were moved to Solr distribution and could be used even in
the next major version 7.
* TrackingIndexWriter functionality has been folded into IndexWriter in
6.2. Its APIs now return a long sequence number which can be passed to
ControlledRealTimeReopenThread directly without TrackingIndexWriter
acting as a mediator.
Feature: Issue 7856
Change-Id: I25e8454e2b541eb5d8aac1e98d011c65b5e12240
e9bcbadd5a