v0.8.20: critical bug fixes

Bug Fixes

• fix: add shutdown to async loader by @chebbyChefNEQ in #1690
• fix: use eplison to handle all zero cosine values by @eddyxu in #1696
• fix: prevent stats meta from breaking old readers by @wjones127 in #1699
• fix: add _rowid when use_stats=False by @wjones127 in #1700
• perf: revert back to hashmap by @chebbyChefNEQ in #1692
• fix: remove default memory cap for index training by @wjones127 in #1702

Full Changelog: v0.8.19...v0.8.20