Lock-free loading of relationships

Removes node lock previously used during loading of relationships. This
lock would contend with committing transaction and produce latency issues.
The lock prevented cache poisoning issues introduced by race conditions
between committer and reader(s).

To achieve a lock-free solution some guards had to be added for various
corner cases. These are:
- Committers are now aware of ot he fact that, and parries for,
  concurrent readers loading and caching relationships committed by the
  committer, before the committer has put them into cache itself,
  producing duplicates when the committer would add them.
  (Issue comes from the fact that store is updated before cache).
- Committers uses knowledge about how relationships are stored on disk to
  be able to tell if any reader has loaded the relationships it has just
  written to the store, before updating the cache. This logic sits in a
  filter which is consulted before adding relationships to the cache.
- Nodes evicted from cache as part of being upgraded from sparse to dense
  now happens as part of updating the node instead of as a separate step
  during commit. This to also guard for concurrent readers.
- Simplification and reduction of code and logic of DenseNodeChainPosition
  where all individual chain positions are initialized eagerly, so that
  the filtering logic (as mentioned above) becomes simpler.
- Ordering of relationship-related records when applying transaction
  to store changed to: relationships, relationship groups, nodes so that
  additions to relationship chains are seen atomically by
  concurrent readers.
Right, the comment said that there was no need to check inUse flag since it being `false` was implicit in the call stack through how we ended up in this method. That isn't true, and given the different call sites, the inUse flag can be false in some, true in some. When the code was first written it may have been the case that the only call site was from a place where we knew that inUse was false. Since that is no longer the case I corrected the comment and the code.

It was merely something I noticed when going around the code there. And even if it didn't make any tests fail it wasn't correct and would probably introduce some issues later on somewhere.