sync.rb - 2 phase lock with counter
  $Release Version: 1.0$
  $Revision$
  by Keiju ISHITSUKA(keiju@ishitsuka.com)

-- Sync_m, Synchronizer_m Usage: obj.extend(Sync_m) or class Foo include Sync_m : end

Sync_m#sync_mode
Sync_m#sync_locked?, locked?
Sync_m#sync_shared?, shared?
Sync_m#sync_exclusive?, sync_exclusive?
Sync_m#sync_try_lock, try_lock
Sync_m#sync_lock, lock
Sync_m#sync_unlock, unlock

Sync, Synchronizer: Usage: sync = Sync.new

Sync#mode
Sync#locked?
Sync#shared?
Sync#exclusive?
Sync#try_lock(mode) -- mode = :EX, :SH, :UN
Sync#lock(mode)     -- mode = :EX, :SH, :UN
Sync#unlock
Sync#synchronize(mode) {...}