chainHead: Ensure reasonable distance between leaf and finalized block (#3562)

This PR ensure that the distance between any leaf and the finalized
block is within a reasonable distance.

For a new subscription, the chainHead has to provide all blocks between
the leaves of the chain and the finalized block.
 When the distance between a leaf and the finalized block is large:
 - The tree route is costly to compute
 - We could deliver an unbounded number of blocks (potentially millions)
(For more details see
https://github.com/paritytech/polkadot-sdk/pull/3445#discussion_r1507210283)

The configuration of the ChainHead is extended with:
- suspend on lagging distance: When the distance between any leaf and
the finalized block is greater than this number, the subscriptions are
suspended for a given duration.
- All active subscriptions are terminated with the `Stop` event, all
blocks are unpinned and data discarded.
- For incoming subscriptions, until the suspended period expires the
subscriptions will immediately receive the `Stop` event.
    - Defaults to 128 blocks
- suspended duration: The amount of time for which subscriptions are
suspended
    - Defaults to 30 seconds
 
 
 cc @paritytech/subxt-team

---------

Signed-off-by: Alexandru Vasile <alexandru.vasile@parity.io>
Co-authored-by: Sebastian Kunert <skunert49@gmail.com>