Multireader Support in Searcher Manager

[Shibi-bala]

Description

Copied from #13975

I'd like to use MultiReader inside my searcher manager, but currently there is only support for DirectoryReader. Not sure about the context for. why this was the case initially, seems specific to some desired use-case https://issues.apache.org/jira/browse/LUCENE-6087.

Anyway should be a 1-line change here:

lucene/lucene/core/src/java/org/apache/lucene/search/SearcherManager.java

Line 125 in 9e90fb5

public SearcherManager(DirectoryReader reader, SearcherFactory searcherFactory)

I found a relevant stack overflow question here:
https://stackoverflow.com/questions/49817453/searchermanager-and-multireader-in-lucene

[mikemccand]

Thanks @Shibi-bala -- I agree it's odd it was scoped to just DirectoryReader -- any IndexReader should work as long as it can openIfChanged on itself.

I think English.java (from Lucene's test-framework) was maybe deleted long ago? Maybe simplify the test to not bother with English words... just Integer.toString(i) or "" + i should be fine?

Also, please revert the wildcard import (import org.apache.lucene.index.*) -- I think our style checker (jtidy/spotless) will be unhappy with that.