Writing of state file(s) should be atomic

[mbartosch]

If CertNanny is interrupted while writing the state file this may lead to a non-recoverable error. On next invocation CertNanny will try to re-instantiate the state file and will fail due to a defective state file.

Suggested remedy.

1. Do not fail if a read error on the state file is encountered. Instead delete state and all corresponding files (key, request)
2. When writing the state file first write the data to a temporary file. After successful completion atomically rename the temp file to the target state file, overwriting this file.