DataSvc Recovery

[yamingk]

Add Data Service Recovery and its test case.

The recovery flow for data service is(most of the logic are already there):

load_vdev -> open_vdev
vdev->add_chunk(...) -> create_blk_allocator for each chunk, each blk allocator register meta_blk_found callback for every possible meta blks possibly written before recovery boot.
Start meta blk service
Each BlkAllocator instance receives meta blks from MetaSvc (via pre-registered callback), and load them into memory.
Recovery done.
Start serving I/O requests;
A new test case added with alloc/write/free/cp_flush, then recover, followed by another round of alloc/write/cp.

Test case added and passed.

[codecov-commenter]

Codecov Report

Patch coverage: 65.00% and project coverage change: +1.67% 🎉

Comparison is base (edf8932) 48.21% compared to head (012069f) 49.89%.

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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #178      +/-   ##
==========================================
+ Coverage   48.21%   49.89%   +1.67%     
==========================================
  Files          86       86              
  Lines        7317     7314       -3     
  Branches      959      958       -1     
==========================================
+ Hits         3528     3649     +121     
+ Misses       3408     3273     -135     
- Partials      381      392      +11     

Files Changed	Coverage Δ
src/include/homestore/blk.h	71.73% <ø> (ø)
src/lib/blkalloc/append_blk_allocator.h	100.00% <ø> (ø)
src/lib/device/virtual_dev.cpp	45.08% <22.22%> (+2.61%)	⬆️
src/lib/blkalloc/append_blk_allocator.cpp	62.33% <100.00%> (+9.77%)	⬆️

... and 12 files with indirect coverage changes

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