Increase LevelDB max_open_files unless on 32-bit Unix.
This change significantly increases IBD performance by increasing the amount of the UTXO index that can remain in memory. To ensure this doesn't cause problems in the future, a static_assert on the LevelDB version has been added, which must be updated by anyone upgrading LevelDB.
Backport of core PR12495
Test Plan: Run IBD on CI (and expect a significant performace improvement).
Reviewers: #bitcoin_abc, deadalnix, fpelliccioni
Reviewed By: fpelliccioni
Differential Revision: https://reviews.bitcoinabc.org/D4000