Fix subtle concurrency bugs in ddsrt_init(), ddsrt_fini()

[mvandenhoek]

This fixes subtle concurrency bugs in ddsrt_init() and ddsrt_fini().

The first problem is that ddsrt_init() has a ‘goto retry_init’ which results in the refcount being incremented more than once, which means that with balanced ddsrt_init() and ddsrt_fini() calls, the actual finalization will never be done. This happens even if you strictly orchestrate the threads such that the first ddsrt_fini() will only happen after all calls to ddsrt_init() have completed, so that ddsrt_init() and ddsrt_fini() are never called concurrently with one another.

The second problem occurs when ddsrt_init() and ddsrt_fini() are called concurrently. When the last ddsrt_fini() call happens, concurrent ddsrt_init() calls will end up being blocked indefinitely, so the program just hangs.

My fix solves both problems:

• Remove the retry logic to avoid extra increments that mess with the refcount balance
• Correctly handle concurrent calls to ddsrt_init() and ddsrt_fini()