Stability improvement

This release is a pure stability patch, and should mitigate some issues observed during testing:

• When subjected to stress, the firmware sometimes shortcuts the handle cache linked list, excluding some handles from the pool of handles being transmitted. This is observable in the ping-pong example project if run with more than 10 devices: Devices stop getting updates from the central device one by one, until there is only one device left getting pings on its pongs. This was caused by a race condition, as the version handler had a hole in its thread safety. All manipulation of the cache-linked list is now serialized with the event-handler. This removed all symptoms from all our test cases, and the mesh performs much better under pressure.
• The trickle timers for each handles had a possiblity of falling behind, leading to devices not being able to catch up with its timers. This issue only appeared when the mesh was tested under heavy stress (20+ devices within range of each other), and looked like non-stop spamming of packets. This issue is now mitigated, as the trickle timer is forced into a "future" timeout if the device catches up with a whole interval.
• The bootloader "early-exit" check was moved to after the init-process, as the journal module had to be initialized to make sure that we weren't in the middle of a transaction.