Fix: First Commit Win guarantee may be broken under high concurrency

[xuerongNanopay]

Thank you for submitting this pull request. We really appreciate you spending the time to work on SurrealDB. 🚀 🎉

What is the motivation?

The first-committer-wins guarantee may be broken under high concurrency.
The root cause is that the write-write conflict check doesn't not operate under serialization manner.

What does this change do?

1. move validate_write_conflicts method into CommitEnv trait.
2. use batch.starting_seq_num to store visible_seq_num for validate_write_conflicts initially, and will be updated to log sequence number before write to WAL
3. Operate validating write-write conflict, WAL writing, and memtable insertion under write_mutex lock

What is your testing strategy?

Run tests in my local. All passed.

Is this related to any issues?

#376

Does this change need documentation?

• No documentation needed

Does this change make any alterations to environment variables or CLI commands?

• No changes made to env vars

Have you read the Contributing Guidelines?

• I have read the Contributing Guidelines

[arriqaaq]

Sorry for the delay in merging this. LGTM. I'll run some benchmarks and confirm