bitfield_distribution: Move on blocking pool and use custom capacity (#5787)
## Description
Details and rationale explained here:
https://github.com/paritytech/polkadot-sdk/issues/5657#issuecomment-2363076080
Fixes: https://github.com/paritytech/polkadot-sdk/issues/5657
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Signed-off-by: Alexandru Gheorghe <[email protected]>