Implement new event API base of Future framework

[Ingvord]

Event API can greatly benefit from using standard Future API:

Future<?> future = attribute.subscribe();

future.get(); //blocks till event comes or throws exception

[codecov[bot]]

Codecov Report

Merging #42 into master will increase coverage by 0.01%. The diff coverage is n/a.

@@             Coverage Diff              @@
##             master      #42      +/-   ##
============================================
+ Coverage     25.47%   25.49%   +0.01%     
+ Complexity     1503     1502       -1     
============================================
  Files           302      301       -1     
  Lines         20712    20703       -9     
  Branches       2342     2341       -1     
============================================
+ Hits           5277     5278       +1     
+ Misses        14972    14963       -9     
+ Partials        463      462       -1

Impacted Files	Coverage Δ	Complexity Δ
...rg/tango/server/servant/AttributeGetterSetter.java	28.04% <0%> (ø)	19% <0%> (ø)	:arrow_down:
.../org/tango/server/cache/NoCacheFoundException.java
...java/org/tango/server/cache/TangoCacheManager.java	55.83% <0%> (+0.78%)	30% <0%> (-1%)	:arrow_down:
.../java/org/tango/server/dynamic/DynamicManager.java	47% <0%> (+0.79%)	11% <0%> (ø)	:arrow_down:
...in/java/org/tango/server/cache/PollingManager.java	46.82% <0%> (+1.05%)	21% <0%> (ø)	:arrow_down:

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[Ingvord]

Can be closed now. No longer relevant