add async version of append

[lukaszstolarczuk]

using libminiasync.

It's an early draft with still some extra features to add, but it seems to compile and execute (an example) just fine.

• [x] wait for https://github.com/pmem/miniasync/pull/69
• [x] wait for https://github.com/pmem/pmemstream/pull/154 && update example

---

This change is 

[codecov-commenter]

Codecov Report

Merging #136 (7534e37) into master (f03bd0a) will decrease coverage by 0.21%. The diff coverage is 82.92%.

@@            Coverage Diff             @@
##           master     #136      +/-   ##
==========================================
- Coverage   91.88%   91.66%   -0.22%     
==========================================
  Files          13       13              
  Lines         739      780      +41     
==========================================
+ Hits          679      715      +36     
- Misses         60       65       +5     

Flag	Coverage Δ
tests_clang_debug_cpp17	92.43% <82.92%> (-0.54%)	:arrow_down:
tests_gcc_debug_cpp17	86.80% <82.50%> (+0.63%)	:arrow_up:

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
src/libpmemstream.c	89.91% <82.92%> (-1.54%)	:arrow_down:
src/region.c	93.60% <0.00%> (+1.59%)	:arrow_up:

:mega: Codecov can now indicate which changes are the most critical in Pull Requests. Learn more

[lukaszstolarczuk]

I've rebased the PR and updated the example to run on multiple regions.