[#2888] Fix bvar worker starvation with recursive bthread mutex

[walterzhaoJR]

Replace the recursive pthread mutex protecting bvar's VarMap with a bthread-aware recursive mutex so yielding metric callbacks do not block worker pthreads. Add regression coverage for worker starvation, recursive ownership across bthread migration, pthread use, and high contention.

Related issue: #2888

What problem does this PR solve?

Issue Number: resolves #2888

Problem Summary:

bvar::Variable protects its global VarMap with a recursive pthread mutex. If a metric callback, such as a PassiveStatus getter, yields while holding this lock, other bthreads may block their worker pthreads while waiting for it. Once all workers are blocked, the lock holder cannot resume, causing worker starvation or deadlock.

A pthread recursive mutex also identifies its owner by OS pthread, which does not correctly represent a bthread that may migrate between workers.

What is changed and the side effects?

Changed:

• Add bthread_recursive_mutex_t and its C++ wrapper, bthread::RecursiveMutex.
• Track ownership by bthread ID for bthreads while retaining support for native pthread callers.
• Use bthread_mutex_t for contention so waiting bthreads are suspended instead of blocking worker pthreads.
• Replace the recursive pthread mutex protecting bvar's VarMap with bthread::RecursiveMutex.
• Add regression tests covering:
  • bvar worker starvation;
  • recursive ownership across bthread yields and worker migration;
  • native pthread and C++ wrapper usage;
  • high-contention recursive locking.
• Keep the standalone test_bvar target independent of the full bthread runtime.

Side effects:

• Performance effects:

  • Recursive lock operations add small owner and recursion-depth checks.
  • Under contention, logical bthreads are suspended instead of blocking worker pthreads.
  • The stress test completes 640,000 critical-section operations while exercising frequent cross-worker migration.

• Breaking backward compatibility:

  • No. This change adds new APIs and only replaces an internal lock used by bvar's VarMap.

Test results

• bthread_mutex_unittest: 3 recursive mutex tests passed.
• brpc_bvar_mutex_unittest: 3 bvar regression tests passed.
• test_bvar: 64/64 tests passed.
• All related targets compiled successfully with GCC 10.

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Check List:

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