memiavl/tree is not thread safe

because iavl.Cache is not thread safe. we may need to consider `ApplyChangeSetAsync` and `WaitToCompleteAysncWrite` if we need to run async writers, now we use unchecked in-place updates.

Start by reading memiavl/tree and the iavl.Cache usage, then inspect the named ApplyChangeSetAsync and WaitToCompleteAysncWrite paths. Determine how the current unchecked in-place updates interact with concurrent access and whether asynchronous writers are required. Done means the thread-safety behavior and needed change are clearly established.

Written by the indexing model from the issue text.