When compiled with NTS support, don't require a SIV cipher to be always
supported (e.g. due to a different version of a library used for
building). Handle this case with a fatal message instead of crash.
Also, check the support early in the client unit test to prevent a hang.
Add a context structure for the algorithm and keys established by
NTS-KE. Modify the client to save the context and reset the SIV key to
the C2S/S2C key before each request/response instead of keeping two SIV
instances.
This will make it easier for the server to support different algorithms
and allow the client to save the context with cookies to disk.
Make the NTS-KE retry interval exponentially increasing, using a factor
provided by the NKE session. Use shorter intervals when the server is
refusing TCP connections or the connection is closed or timing out
before the TLS handshake.
