[Investigate/Testing] Add test-case to verify that changing `RetentionPeriod` (+/-) live does not break anything

[bkontur]

Now we have RetentionPeriod as storage item exposed by runtime API, so the node binaries are reading it directly, no hard-coded values anymore.

We should have some kind of test (or at least verify), probably on the runtime level or maybe pallet level is enough?

Something like:

RetentionPeriod = 3
Block1
- store data
Block2
- store data
Block3
- store data
Block4
- check_proof(Block1) inherent
- store data
- change RetentionPeriod = 2 (decrease should be fine)
Block5
- check_proof(Block3) inherent
- store data
- change RetentionPeriod = 4 (increase, potential problems?)
Block6
- check_proof(Block2) inherent (Block2 transaction metadata where removed ??)
  - we could have a problem here, because transaction metdata were already removed?, do we need here some more fixing - something like to store previous retention period from some block and do some magic?) 

The problem could be also when we just do a decrease, there could exist potentially some gap between, which is never removed from transaction metadata. (probably not a big issue, we can cleanup manually when decreasing)

[bkontur]

@karolk91 I think we can also test that "Data fetching from other collators" with this RetentionPeriod, for example:

• we run network with RetentionPeriod=20 blocks
• start another collatorA
• filling the blocks
• stop collatorA at some point
• start new fresh collatorB and let it sync from scratch
• start collatorA again, should fetch the GAP

or something like that