Header is used to notify the client when a connection to zookeeper has been lost and there is a possibility of stale data on the node the request is coming from.
* This increases <mergePolicy> vs. <mergePolicyFactory> test coverage.
* This temporarily also adds a little test setup complexity but when SOLR-8668 removes support for <mergePolicy> then test setup will return to its original complexity.
Here in SOLR-8621:
* UpgradeIndexMergePolicyFactory extends WrapperMergePolicyFactory
* (WrapperMergePolicyFactoryTest's) DefaultingWrapperMergePolicyFactory extends WrapperMergePolicyFactory
Elsewhere in SOLR-5730:
* SortingMergePolicyFactory will extend WrapperMergePolicyFactory
* fix so that getMergePolicy() can now be called more than once
* added WrapperMergePolicyFactoryTest.testUpgradeIndexMergePolicyFactory()
* account for overlap between wrapping and wrapped setters (and disallow it)
** illustration:
<mergePolicyFactory class="UpgradeMergePolicyFactory">
<int name="noCFSRatio">0.24</int>
<str name="wrapped.prefix">mergePolicy</str>
<str name="mergePolicy.class">TieredMergePolicyFactory</str>
<int name="mergePolicy.noCFSRatio">0.42</int>
</mergePolicyFactory>
** implementation details: the wrapping MP's setter calls the wrapped MP's setter and in the current code the wrapping MP's value prevails i.e. the 0.24 value in the illustration since the wrapped MP is constructed before the wrapping MP. an end-user however might reasonably assume that the wrapped MP's 0.42 value will prevail. at best configuring the same setter twice within the same overall <mergePolicyFactory> element is ambiguous and so the code now disallows it.
* added bad-mpf-solrconfig.xml as MergePolicyFactory equivalent of bad-mp-solrconfig.xml (with DummyMergePolicyFactory as equivalent to DummyMergePolicy)
* added solrconfig-logmergepolicyfactory.xml as MergePolicyFactory equivalent of solrconfig-logmergepolicy.xml
* added solrconfig-mergepolicyfactory-nocfs.xml as MergePolicyFactory equivalent of solrconfig-mergepolicy-nocfs.xml
* added solrconfig-indexconfig-mergepolicyfactory.xml as MergePolicyFactory equivalent of solrconfig-indexconfig.xml
* added solrconfig-warmer-randommergepolicyfactory.xml as MergePolicyFactory equivalent of solrconfig-warmer.xml
