fix: rename close_db_connection to flush

[rymnc]

No description provided.

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Benchmark for b4c945b

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Test	Base	PR	%
FullMerkleTree::delete	1294.6±43.82ns	1443.3±40.00ns	+11.49%
FullMerkleTree::get	0.0±0.00ns	0.0±0.00ns	NaN%
FullMerkleTree::override_range	3.3±0.09µs	3.6±0.15µs	+9.09%
FullMerkleTree::set	1313.7±55.27ns	1290.1±26.55ns	-1.80%
OptimalMerkleTree::compute_root	1626.8±56.92ns	1458.8±29.94ns	-10.33%
OptimalMerkleTree::delete	1642.9±85.45ns	1458.8±72.10ns	-11.21%
OptimalMerkleTree::get	32.8±1.27ns	34.5±4.55ns	+5.18%
OptimalMerkleTree::override_range	8.0±0.34µs	7.5±0.35µs	-6.25%
OptimalMerkleTree::set	1545.2±78.31ns	1440.7±44.83ns	-6.76%

[github-actions]

Benchmark for b4c945b

Click to view benchmark

Test	Base	PR	%
Pmtree::compute_root	2.0±0.02ns	2.0±0.01ns	0.00%
Pmtree::get	547.9±6.19ns	543.9±8.66ns	-0.73%
Pmtree::override_range	203.1±5.97µs	204.1±5.33µs	+0.49%
Pmtree::set	83.2±0.70µs	83.8±0.65µs	+0.72%
Pmtree:delete	83.2±0.71µs	83.5±0.75µs	+0.36%