py9p: support rename() routine
py9p: flush cache entries on rename
py9p: rewrite read/write routines

 * thread safe: each read/write cycle should be done sequentially
 * flush directory cache entry on write ('cause size changes)
 * use per-Fid() iounit + default iounit for fidcache
Merge branch 'master' into package-alt
1.0.7-alt2: version update