Improve socket options

[mschubert]

• Remove ZeroMQ<3 code, this will never be compiled due to static version assert >= 3
• Replace redundant option setting functions by setSockOptChr and setSockOptInt
• Expose high water mark option for ZeroMQ>=3 using set.send.hwm and set.rcv.hwm
• Include TCP keep-alive options supported by ZeroMQ>=3 using set.tcp.keepalive

[codecov-io]

Codecov Report

Merging #49 into master will increase coverage by 2.88%.
The diff coverage is 0%.

@@            Coverage Diff            @@
##           master     #49      +/-   ##
=========================================
+ Coverage   33.72%   36.6%   +2.88%     
=========================================
  Files           4       4              
  Lines         685     631      -54     
=========================================
  Hits          231     231              
+ Misses        454     400      -54

Impacted Files	Coverage Δ
src/interface.cpp	31.68% <0%> (+2.97%)	⬆️
R/rzmq.R	23.21% <0%> (+2.24%)	⬆️

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