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%)	:arrow_up:
R/rzmq.R	23.21% <0%> (+2.24%)	:arrow_up:

---

Continue to review full report at Codecov.

Legend - Click here to learn more Δ = absolute <relative> (impact), ø = not affected, ? = missing data Powered by Codecov. Last update b1779cf...50d4165. Read the comment docs.