Fix data race in TPE

[COLDTURNIP]

Reference Issues/PRs

Fixes #236

What does this implement/fix? Explain your changes.

While TPE reviews the trial parameters before making suggestions, the historical internal parameter should be protected under storage's mutex.

[c-bata]

@COLDTURNIP Thank you for reporting the bug and creating a patch.

I have recently resumed maintenance and upon reviewing your PR, I noticed that while it does address the issue at hand, it could potentially cause an increase in SQL queries when using the RDB backend, which might lead to a degradation in performance.

To address this, I have prepared a separate PR (#246) that tackles the same issue but without additional SQL queries, ensuring a better performance. I am grateful for your understanding and once again, thank you for your input and dedication.