fix(unicylce cleanup): defer cleanup until vehicle callbacks complete

[florinm03]

Fixes #218.

Basically, when walking mode exits, walk.lua does this:

local unicycle = getCurrentUnicycle()
be:enterVehicle(0, vehicle)
unicycleId = unicycle:getId()

The multiplayer vehicle manager then loops through all vehicles and deletes every unicycle whose ID is not new_id.

walk.lua still had the Lua variable referring to the old unicycle and called:

unicycle:getId()

The Lua variable still existed, but the underlying BeamNG vehicle object had already been destroyed. That produced the invalid userdata error.

The fix moves the deletion to the next update tick so that the game doesn't call non-existent objects.

Tested it on a few new restarts of the game and mod and it did not happen anymore. :)

Errors: