Packaging: Check if proc file exists before calling sysctl

The packaged init scripts could return an error, if the file
/proc/sys/vm/max_map_count was not existing and we still called
sysctl.

This is primarly to prevent confusing error messages when elasticsearch
is started under virtualized environments without a proc file system.

Closes #4978
Scripting: Add explicit error message when script_score script returns NaN

When a scoring script returns not a number, the current message is confusing (IllegalArgumentException[docID must be >= 0 and < maxDoc=3 (got docID=2147483647)]). This commit adds the error message ScriptException[script score function returns a wrong score: NaN].

Closes #2426
Add more trace loggin to LocalIndexShardGateway.java
[STORE] Expose ShardId via LeafReader rather than Direcotry API

Today we try to fetch a shard Id for a given IndexReader / LeafReader
by walking it's tree until the lucene internal SegmentReader and then
casting the directory into a StoreDirecotory. This class is fully internal
to Elasticsearch and should not be exposed outside of the Store.

This commit makes StoreDirectory a private inner class and adds dedicated
ElasticsearchDirectoryReader / ElasticserachLeafReader exposing a ShardId
getter to obtain information about the shard the index / segment belogs to.

These classes can be used to expose other segment specific information in
the future more easily.
[TEST] Add unit tests for DiskThresholdDecider settings