bugfix: reqstat record keys may duplicate if concurrency level is high

[Chion82]

In ngx_http_reqstat_module:

• Root of the red-black tree may change when inserting a new record if rotation occurs. ctx->sh->rbtree.root is NOT read-only.
• The process of retrieving the tree root is not protected by mutex lock, which may introduce race condition.

In some rare cases, when the concurrency level is high enough,
ngx_http_reqstat_rbtree_lookup() may fail to find an existing node and allocate a new one. This will cause repeated keys in the reqstat output.

[CLAassistant]

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