Writes debug files by default to a hard-coded location and doesn't remove them

[deadok22]

Since https://github.com/DataDog/dd-opentracing-cpp/pull/147, the library writes some debug information to a /var/tmp/dd-opentracing-cpp/startup_options-*.json file on startup. This is problematic:

• The users risk running out of disk in environments where the processes consuming the library are started frequently (this did actually happen in my setup with NGINX Ingress Controller)
• The hard-coded location is bad for portability

I believe this output should be turned off by default and its destination should be made configurable.