Add protection for concurrent access to m_down_buffers

[koujl]

Issue: #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.

[codecov-commenter]

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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 ⚠️
src/lib/index/index_cp.cpp	75.00%	0 Missing and 1 partial ⚠️
src/lib/index/wb_cache.cpp	85.71%	1 Missing ⚠️

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Additional details and impacted files

@@             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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[zhiteng]

The PR message should be put into commit message as well with some modification, which the reference to eBay internal JIRA isn't suitable for public github.com hosted open source project. My suggestion is to create an issue and refer to that issue instead of JIRA. That way people from outside of eBay may see the entirety of whole issue along with this fix.

[koujl]

Linked to #578

[shosseinimotlagh]

LG!
Todo: needs to change to set