[Bugfix] Prepare failed when the transaction still in processing

[gohalo]

What type of PR is this：

• BugFix
• Feature
• Enhancement
• Refactor
• UT
• Doc
• Tool

Which issues of this PR fixes ：

Fixes #383

Problem Summary(Required) ：

When invoking the prepare method for some transaction, there maybe get some chance failed, with the following code.

Status TransactionMgr::commit_transaction(const HttpRequest* req, std::string* resp) {
    // ......
        if (!ctx->lock.try_lock()) {
            st = Status::TransactionInProcessing("Transaction in processing, please retry later");
            *resp = _build_reply(label, TXN_COMMIT, st);
            return st;
        }
    // ......
}

And the flink connector just throw an exception without retry.

Checklist:

• I have added test cases for my bug fix or my new feature
• This pr will affect users' behaviors
• This pr needs user documentation (for new or modified features or behaviors)
• I have added documentation for my new feature or new function