[Documentation] Add Alcor transaction design document

[pkommoju]

Add document about transactional semantics for Alcor Caches in general
and spefically about Ignite Caches.

[codecov-commenter]

Codecov Report

Merging #611 (183c36a) into master (64f5d83) will increase coverage by 0.02%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##             master     #611      +/-   ##
============================================
+ Coverage     31.98%   32.00%   +0.02%     
- Complexity     1249     1254       +5     
============================================
  Files           525      525              
  Lines         13439    13439              
  Branches       1657     1657              
============================================
+ Hits           4298     4301       +3     
- Misses         8555     8556       +1     
+ Partials        586      582       -4     

Impacted Files	Coverage Δ	Complexity Δ
...alcor/portmanager/util/RestParameterValidator.java	43.07% <0.00%> (-3.08%)	13.00% <0.00%> (-1.00%)
...alcor/elasticipmanager/dao/ElasticIpAllocator.java	63.81% <0.00%> (-0.28%)	48.00% <0.00%> (-1.00%)
...lcor/portmanager/processor/DataPlaneProcessor.java	70.27% <0.00%> (+0.54%)	42.00% <0.00%> (+3.00%)
...rewei/alcor/portmanager/processor/PortContext.java	71.42% <0.00%> (+3.89%)	35.00% <0.00%> (+3.00%)
...i/alcor/portmanager/processor/RouterProcessor.java	85.29% <0.00%> (+5.88%)	10.00% <0.00%> (+1.00%)

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[xieus]

@pkommoju A few quick comments on formatting. Will look deeper.