client: refactoring of asyn-write specific options. There now is an...

client: refactoring of asyn-write specific options. There now is an enable-async-writes instead of implicitly enabling by setting max-writeahead > 0. The latter option was removed, instead there are async-writes-max-reqs (formerly known as max-writeahead-requests) and async-writes-max-req-size-kB (which is an advanced option and thus hidden).

git-svn-id: https://xtreemfs.googlecode.com/svn/trunk@3604 f8528748-c2c0-11dd-ba5c-d3b7c2b3b534