IndexOutput.writeString() should write length in bytes

We should change the format of strings written to indexes so that the length of the string is in bytes, not Java characters. This issue has been discussed at:

http://www.mail-archive.com/java-dev@lucene.apache.org/msg01970.html

We must increment the file format number to indicate this change. At least the format number in the segments file should change.

I'm targetting this for 2.1, i.e., we shouldn't commit it to trunk until after 2.0 is released, to minimize incompatible changes between 1.9 and 2.0 (other than removal of deprecated features).