Increased open files usage in upcoming 0.11

Hi,

Just pushed this one:
http://github.com/elasticsearch/elasticsearch/issues/issue/384. It means
faster indexing and faster recovery, though does require more open files to
be set. Hopefully, most people have already set pretty high limits for the
open files, so we should be ok.

p.s. If someone knows how to set the open files in the elasticsearch.sh
script in a cross platform manner it would be great (or maybe just print a
warning if its less than, lets say, 8k?).

-shay.banon