rpl: fixed bug in debug msg introduced by my earlier changes
rpl: remove rpl_send_mutex

This PR is based on #2155.
Since RPL/Trickle is only using one thread in #2155, the send mutex becomes
obsolete and can be removed safely.
rpl: multiple instances

Conflicts:
	sys/net/routing/rpl/rpl_dodag.c
	sys/net/routing/rpl/rpl_storing/rpl_storing.c
rpl: multiple instances for nonstoring mode, too
multiple dodags for storing mode, mostly
check for root before deciding if to switch dodags
reset trickle timer on reception of DIS
send DIS before preferred parent is timed out
added check for instance id when deleting parents
check instance when deleting parent. more cleanup