Make C++ safe

[richfitz]

There are a couple of issues here:

• I see a compilation warning due to potentially problematic code due to combining hiredis and C++:

In file included from /usr/include/hiredis/hiredis.h:40,
                 from heartbeat.h:8,
                 from thread.h:4,
                 from thread.cpp:1:
/usr/include/hiredis/sds.h:48:10: warning: ISO C++ forbids flexible array member ‘buf’ [-Wpedantic]
   48 |     char buf[];
      |          ^~~
/usr/include/hiredis/sds.h:54:10: warning: ISO C++ forbids flexible array member ‘buf’ [-Wpedantic]
   54 |     char buf[];
      |          ^~~
/usr/include/hiredis/sds.h:60:10: warning: ISO C++ forbids flexible array member ‘buf’ [-Wpedantic]
   60 |     char buf[];
      |          ^~~
/usr/include/hiredis/sds.h:66:10: warning: ISO C++ forbids flexible array member ‘buf’ [-Wpedantic]
   66 |     char buf[];
      |          ^~~
/usr/include/hiredis/sds.h:72:10: warning: ISO C++ forbids flexible array member ‘buf’ [-Wpedantic]
   72 |     char buf[];
      |          ^~~

This would be eliminated by using C everywhere, which might involve moving to use https://tinycthread.github.io/ for the threading.

• The C++ might throw and that will end poorly! We need to put all the C++ code into a try/catch block
• We could rewrite the whole package in C++ using cpp11, but that leaves the first issue unresolved