prospective-parachains rework (#4035)

Reworks prospective-parachains so that we allow a number of unconnected
candidates (for which we don't know the parent candidate yet). Needed
for elastic scaling:
#3541. Without this,
candidate B will not be validated and backed until candidate A (its
parent) is validated and a backing statement reaches the validator.

Due to the high complexity of the subsystem, I rewrote parts of it so
that we don't concern ourselves with candidates which form cycles or
which form parachain forks. We now have "Fragment chains" instead of
"Fragment trees". This greatly simplifies some of the code and is a
compromise we can make. We just need to make sure that cycle-producing
parachains don't brick the relay chain and that fork-producing
parachains can still make some progress (on one core at least). The only
forks that are allowed are those on the relay chain, obviously.

Unconnected candidates are kept in the `CandidateStorage` and whenever a
new candidate is introduced, we try to repopulate the chain with as many
candidates as we can.

Also fixes #3219

Guide changes will be done as part of:
#3699

TODOs:

- [x] see if we can replace the `Cow` over the candidate commitments
with an `Arc` over the entire `ProspectiveCandidate`. It's only being
overwritten in unit tests. We can work around that.
- [x] finish fragment_chain unit tests
- [x] add more prospective-parachains subsystem tests
- [x] test with zombienet what happens if a parachain is creating cycles
(it should not brick the relay chain).
- [x] test with zombienet a parachain that is creating forks. it should
keep producing blocks from time to time (one bad collator should not DOS
the parachain, even if throughput decreases)
- [x] add some more logs and metrics
- [x] add prdoc and remove the "silent" label

---------

Signed-off-by: Andrei Sandu <andrei-mihail@parity.io>
Co-authored-by: Andrei Sandu <andrei-mihail@parity.io>