Use mmapfs as default store type

With this commit we switch the default store type from `hybridfs` to `mmapfs`.
While `hybridfs` is beneficial for random access workloads (think: updates and
queries) when the index size is much larger than the available page cache, it
incurs a performance penalty on smaller indices that fit into the page cache (or
are not much larger than that).

This performance penalty shows not only for bulk updates or queries but also for
bulk indexing (without *any* conflicts) when an external document id is provided
by the client. For example, in the `geonames` benchmark this results in a
throughput reduction of roughly 17% compared to `mmapfs`. This reduction is
caused by document id lookups that show up as the top contributor in the profile
when enabling `hybridfs`. Below is such an example stack trace as captured by
async-profiler during a benchmarking trial where we can see that the overhead is
caused by additional `read` system calls for document id lookups:

```
__GI_pread64
sun.nio.ch.FileDispatcherImpl.pread0
sun.nio.ch.FileDispatcherImpl.pread
sun.nio.ch.IOUtil.readIntoNativeBuffer
sun.nio.ch.IOUtil.read sun.nio.ch.FileChannelImpl.readInternal
sun.nio.ch.FileChannelImpl.read
org.apache.lucene.store.NIOFSDirectory$NIOFSIndexInput.readInternal
org.apache.lucene.store.BufferedIndexInput.refill
org.apache.lucene.store.BufferedIndexInput.readByte
org.apache.lucene.store.DataInput.readVInt
org.apache.lucene.store.BufferedIndexInput.readVInt
org.apache.lucene.codecs.blocktree.SegmentTermsEnumFrame.loadBlock
org.apache.lucene.codecs.blocktree.SegmentTermsEnum.seekExact
org.elasticsearch.common.lucene.uid.PerThreadIDVersionAndSeqNoLookup.getDocID
org.elasticsearch.common.lucene.uid.PerThreadIDVersionAndSeqNoLookup.
lookupVersion
org.elasticsearch.common.lucene.uid.VersionsAndSeqNoResolver.loadDocIdAndVersion
org.elasticsearch.index.engine.InternalEngine.resolveDocVersion
org.elasticsearch.index.engine.InternalEngine.planIndexingAsPrimary
org.elasticsearch.index.engine.InternalEngine.indexingStrategyForOperation
org.elasticsearch.index.engine.InternalEngine.index
org.elasticsearch.index.shard.IndexShard.index
org.elasticsearch.index.shard.IndexShard.applyIndexOperation
org.elasticsearch.index.shard.IndexShard.applyIndexOperationOnPrimary
[...]
```

For these reasons we are restoring `mmapfs` as the default store type.

Relates elastic#36668

docs/reference/index-modules/store.asciidoc

@@ -40,7 +40,7 @@ The following sections lists all the different storage types supported.
 `fs`::
 
 Default file system implementation. This will pick the best implementation
-depending on the operating environment, which is currently `hybridfs` on all
+depending on the operating environment, which is currently `mmapfs` on all
 supported systems but is subject to change.
 
 [[simplefs]]`simplefs`::

server/src/main/java/org/elasticsearch/index/IndexModule.java

@@ -357,7 +357,7 @@ public interface IndexSearcherWrapperFactory {
 
     public static Type defaultStoreType(final boolean allowMmap) {
         if (allowMmap && Constants.JRE_IS_64BIT && MMapDirectory.UNMAP_SUPPORTED) {
-            return Type.HYBRIDFS;
+            return Type.MMAPFS;
         } else if (Constants.WINDOWS) {
             return Type.SIMPLEFS;
         } else {

server/src/test/java/org/elasticsearch/index/store/IndexStoreTests.java

@@ -77,7 +77,7 @@ private void doTestStoreDirectory(Index index, Path tempDir, String typeSettingV
                     break;
                 case FS:
                     if (Constants.JRE_IS_64BIT && MMapDirectory.UNMAP_SUPPORTED) {
-                        assertHybridDirectory(directory);
+                        assertTrue(type + " " + directory.toString(), directory instanceof MMapDirectory);
                     } else if (Constants.WINDOWS) {
                         assertTrue(directory.toString(), directory instanceof SimpleFSDirectory);
                     } else {