Update callers to SendPacket with std::string's to not devolve to c-strs

Many callers of SendPacket() in RNBRemote.cpp have a local std::string
object, call c_str() on it to pass a c-string, which is then copied into
a std::string temporary object.

Also free JSONGenerator objects once we've formatted them into
ostringstream and don't need the objects any longer, to reduce max
memory use in debugserver.

Differential Revision: https://reviews.llvm.org/D122848
rdar://91117263