Allow switch to start without JSON configuration

[antoninbas]

The target binaries can be started without an input JSON config by using
the --no-p4 command line flag. Until a config has been pushed to the
switch however, no traffic can be processed. The start_and_return method
was modified to block until a config is pushed. The virtual method that
needs to be implemented by switches was renamed
start_and_return_. Similarly the receive virtual method was also renamed
receive_. This means that existing target switches outside of this repo
will need to be updated.

The tools (e.g. runtime CLI) needed to be updated as a result to support
the empty config case.

[codecov-io]

Codecov Report

Merging #281 into master will increase coverage by 0.06%.

@@            Coverage Diff             @@
##           master     #281      +/-   ##
==========================================
+ Coverage   74.79%   74.85%   +0.06%     
==========================================
  Files         103      103              
  Lines        7589     7620      +31     
==========================================
+ Hits         5676     5704      +28     
- Misses       1913     1916       +3

Impacted Files	Coverage Δ
include/bm/bm_sim/options_parse.h	33.33% <ø> (ø)	✅
include/bm/bm_sim/switch.h	1.39% <100%> (ø)	✅
src/bm_sim/options_parse.cpp	50% <41.66%> (-1.25%)	❌
src/bm_sim/switch.cpp	66.5% <87.87%> (+5.86%)	✅

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