Limit the number of leaves per level

[davxy]

Closes #432

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⚠ DEPENDS ON: paritytech/substrate#12005
(for correct blocks removal from the backend)

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This PR prevents the StateDbError::TooManySiblingBlocks error from being triggered by eagerly removing leaves from the backend on block import and before the error condition is met.

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The strategy is to remove the older block to make space for a new fresh block when we reach a configurable upper bound.

To detect which is the older block at the given level currently we are relying on the implementation of the Backend::leaves() method that ships with substrate:

https://github.com/paritytech/substrate/blob/6b8553511112afd5ae7e8e6877dc2f467850f155/client/api/src/leaves.rs#L204-L206

That is, our implementation already returns the leaves sorted by insertion time (older first).

(Before this, as an alternative - more portable but less elegant - implementation, I was maintaining for every leaf its import order, e.g. its timestamp).

In the end I've preferred to rely on the assumption that the leaves are already given ordered by insertion time to keep it simple.

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If this strategy is good, then I suggest to modify the blockchain Backend trait leaves() method documentation to better fit our ordering assumptions.

https://github.com/paritytech/substrate/blob/6b8553511112afd5ae7e8e6877dc2f467850f155/primitives/blockchain/src/backend.rs#L98-L101

From:

Results must be ordered best (longest, highest) chain first.

To:

Results must be ordered best (longest,highest) chain first and within each level ordered by import time (older first).