Add protection for concurrent access to m_down_buffers

[koujl]

Issue: https://github.com/eBay/HomeStore/issues/578

The down_buf has the real_up_buf (the meta buf) as its up_buffer, but cannot be found in the read_up_buf->m_down_buffers. The inconsistency is caused by unprotected concurrent writes to the vector m_down_buffers.

Add a mutex lock to IndexBuffer as well as extracting add/remove operations into member functions to make the vector thread-safe.

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Codecov Report

Attention: Patch coverage is 85.71429% with 5 lines in your changes missing coverage. Please review.

Project coverage is 67.30%. Comparing base (1a0cef8) to head (76fc41a). Report is 84 commits behind head on master.

Files with missing lines	Patch %	Lines
src/lib/index/index_service.cpp	87.50%	1 Missing and 2 partials :warning:
src/lib/index/index_cp.cpp	75.00%	0 Missing and 1 partial :warning:
src/lib/index/wb_cache.cpp	85.71%	1 Missing :warning:

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #577 +/- ## =========================================== + Coverage 56.51% 67.30% +10.79% =========================================== Files 108 109 +1 Lines 10300 10654 +354 Branches 1402 1454 +52 =========================================== + Hits 5821 7171 +1350 + Misses 3894 2787 -1107 - Partials 585 696 +111 ```

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[koujl]

Linked to https://github.com/eBay/HomeStore/issues/578