Reduce candidates storing time

[kamilsa]

Referenced issues

Closes #2276

Description of the Change

Storing candidates in availability store might lead to OOM in Kusama. This PR attempts reducing timeout for storing candidates improves situation.
In addition candidates will be removed whenever corresponding relay chain blocks are finalized.
It is safe to keep candidates for shorter period as after merging #2271 we are going to store them in db for 25 hours anyway

Possible Drawbacks

None

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