perf: Remove usage of nested lmdb transaction

[chubei]

Spotted this in the flamegraph.

Test method

Tested using data and config of taxi-objectstore in https://github.com/getdozer/dozer-samples-internal/commit/e4b719153a9d82f5fba55bb9637d44cd946f6304. Run following command:

rm -r .dozer; time ~/dozer/target/release/dozer

When trips_cache count reaches 200,000, press Ctrl + C and wait for dozer to quit.

Result

With code in 0c549a2:

~/dozer/target/release/dozer 28.25s user 23.84s system 101% cpu 51.526 total

With code in https://github.com/chubei/dozer/commit/ac46db09aca5b0751b07aaf69bb28acda872ee48:

~/dozer/target/release/dozer 13.62s user 10.19s system 115% cpu 20.578 total

Saves 60%.

[snork-alt]

nice!

[coveralls]

Pull Request Test Coverage Report for Build 4289337951

• 3 of 3 (100.0%) changed or added relevant lines in 1 file are covered.
• 160 unchanged lines in 3 files lost coverage.
• Overall coverage decreased (-0.09%) to 70.571%

---

Files with Coverage Reduction	New Missed Lines	%
dozer-cache/src/cache/lmdb/indexer.rs	40	75.98%
dozer-core/src/record_store.rs	57	82.78%
dozer-sql/src/pipeline/planner/projection.rs	63	60.38%

Totals
Change from base Build 4284179721:	-0.09%
Covered Lines:	27145
Relevant Lines:	38465

---

💛 - Coveralls