Include in the key dump file an identifier, the AEAD number, and the
age of the last key to improve robustness and avoid generating a new key
immediately on start.
Also, improve the code that saves and loads the file.
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.
The server session instances are reused for different clients. Separate
the server name from the label used in log messages and set it on each
start of the session.
Don't require the caller to provide a SCK_Message (on stack). Modify the
SCK_ReceiveMessage*() functions to return a pointer to static buffers,
as the message buffer which SCK_Message points to already is.
