Reenable move semantics and suppress warnings

[mellis13]

This PR makes two small changes to the SmartRedis code:

1. The std::random_device instance is not stored as a member variable on the RedisServer class. The std::random_device is not movable or copyable, and as a result, it was implicitly disabling move semantics for the Client and the RedisServer derivates.
2. Exception class methods have been marked with the appropriate const override keywords to suppress compiler warnings.

[codecov]

Codecov Report

Attention: Patch coverage is 22.22222% with 7 lines in your changes missing coverage. Please review.

Project coverage is 57.62%. Comparing base (4937fbd) to head (589072e).
Report is 15 commits behind head on develop.

Files with missing lines	Patch %	Lines
include/srexception.h	12.50%	7 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##           develop     #520       +/-   ##
============================================
- Coverage    95.92%   57.62%   -38.31%     
============================================
  Files           10      147      +137     
  Lines          884    11708    +10824     
  Branches         0     1127     +1127     
============================================
+ Hits           848     6747     +5899     
- Misses          36     4810     +4774     
- Partials         0      151      +151     

Files with missing lines	Coverage Δ
include/redisserver.h	60.00% <ø> (ø)
src/cpp/redisserver.cpp	98.96% <100.00%> (ø)
include/srexception.h	49.18% <12.50%> (ø)

... and 134 files with indirect coverage changes

[al-rigazzi]

LGTM, thanks for this fix!

[ashao]

Thanks for polishing this