allocator hangs that may occur after calling the C++ API `MallocExtension::instance()->ReleaseFreeMemory()` on some tcmalloc versions/distros.

[ljluestc]

What problem does this PR solve?

• Avoids allocator hangs that may occur after calling the C++ API MallocExtension::instance()->ReleaseFreeMemory() on some tcmalloc versions/distros.
• Adds automated verification to ensure post-release allocations do not hang.

Issue Number: resolves #200

Problem Summary:

• The weak C++ symbol for MallocExtension::instance() may not be reliably overridden in some environments (e.g., Fedora 26), and calling ReleaseFreeMemory() via that path has been observed to hang subsequent allocations on tcmalloc 1.7/2.5.
• brpc already uses the stable C API MallocExtension_ReleaseFreeMemory() at runtime; this PR adds tests to prevent regressions and to make verification straightforward.

What is changed and the side effects?

Changed:

• Add gtest test/brpc_tcmalloc_release_unittest.cpp that:
  • Alloc/free memory → call MallocExtension_ReleaseFreeMemory (weakly referenced) → alloc/free again, with a 10s timeout guard to detect hangs.
• Optionally include the standalone smoke test tcmalloc_release_test.cc (uses a weak symbol so it runs with or without libtcmalloc).

Side effects:

• Performance effects:

  • None at runtime; this PR is test-only. The existing runtime already calls the C API and falls back to malloc_trim on Linux.

• Breaking backward compatibility:

  • None.

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