Thrift C++ runtime uses assert to prevent overflows, checks sanity only in debug builds

Currently there is widespread use of assert in the thrift C++ runtime library. Some of the more disturbing cases are security related, for example checking header sizes. I recommend we eliminate assertions that are only checked in debug mode, and instead throw the appropriate exception, usually a TTransportException with CORRUPTED_DATA as the reason. If we're going to check for an overflow or a buffer overrun, we should do so in debug and release modes. Further, assertions are not easily tested whereas exceptions are.

In THRIFT-3873 apache::thrift::transport::safe_numeric_cast was added, so I also suggest changing static_cast to safe_numeric_cast where appropriate throughout the transport code to catch any overflow errors.

Another location where assert is used liberally is inside the posix Mutex implementation.