Gan Ziheng

iterator: unify behavior and implement `prev` and `to_last` to all Iterators

8

Signed-off-by: GanZiheng Unify all kinds of iterators which implement the `AgateIterator` trait. Suppose we have another two extra elements in the data which the iterator iterates over, one is before...

levels: don't sync directory in memory mode

2

This will cause problem when performing compaction in memory mode, since we will never return tables built by compaction successfully due to the `sync_dir` returning error with empty path input....

As shown in https://github.com/tikv/agatedb/pull/143#issuecomment-1158727895, we may not pass the leak sanitizer test due to the `SEGV` error: Stack trace ``` test ops::transaction_test::normal_db::test_conflict ... LeakSanitizer:DEADLYSIGNAL ==9609==ERROR: LeakSanitizer: SEGV on unknown address...