CHIA-2515 Simplify MempoolItem by constructing SpendBundle on demand from its components

[AmineKhaldi]

Purpose:

Instead of holding a SpendBundle in a MempoolItem, just construct it on-demand from the coin spends and the aggregate signature.

Current Behavior:

We hold coin spends in both spend_bundle and bundle_coin_spends.

New Behavior:

We hold coin spends in bundle_coin_spends and we construct the spend bundle from that and the aggregate signature.

[coveralls-official]

Pull Request Test Coverage Report for Build 18127910299

Details

• 36 of 36 (100.0%) changed or added relevant lines in 8 files are covered.
• 18 unchanged lines in 7 files lost coverage.
• Overall coverage increased (+0.003%) to 91.15%

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Files with Coverage Reduction	New Missed Lines	%
chia/full_node/pending_tx_cache.py	1	96.55%
chia/wallet/util/wallet_sync_utils.py	1	85.57%
chia/consensus/blockchain.py	2	94.38%
chia/rpc/rpc_server.py	2	89.03%
chia/server/node_discovery.py	2	82.11%
chia/wallet/wallet_node.py	4	87.13%
chia/full_node/full_node.py	6	87.27%

Totals
Change from base Build 18104177433:	0.003%
Covered Lines:	102867
Relevant Lines:	112693

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💛 - Coveralls

[arvidn]

looks good.