Round up RM session size to page granularity

Since RM sessions can be used as dataspaces and dataspace sizes are
supposed to have page granularity, RM session sizes should have page
granularity, too.

Fixes #799.
Use one-shot timer in scheduler of Timed_semaphore

Instead of counting jiffies, and sleep periodically, thereby choosing a
specific granularity for timeouts, just program the timer everytime a new
alarm is scheduled, or if the timer triggered, and there are still outstanding
alarms.

Fix #35