2.55 [KK 2009-05-13] 
- Implemented connection error counting of back ends.
- Select-handling revised: atomic readability and writeability checks,
  see xr/fdset/wait.cc and wait_r(), wait_w(), wait_rw().
- Checks for connect() success can be configured to verify only
  writeable status of a socket (see xr/backend/connect.cc, macro
  CONNECTCHECK_ONLY_WRITABLE in xr/etc/Makefile.class).
- Added test/xr-mysql-connect.
- Added commands kill and killstart to xrctl. Action restart renamed
  to stopstart.
- Added simple benchmarking under test/: xr-http-test (based on an
  older script) and xr-smtp-test. Plus a simple analyzer of the
  reported timings, xr-analyze-test.
- Revamped stored-ip dispatcher, added IPStore class to keep track of clients.