refactor(common): LinkedChunk can start by a gap.

This patch removes the invariant stating that a `LinkedChunk` must start
by a chunk of type items. This has never been really useful but it's now
annoying to have this (with iterative loading of a `LinkedChunk` via the
`EventCache`, it's now possible to get a gap as the first chunk). Let's
remove this invariant.

Author: Hywan