nts: refactor NTS context

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.

nts_ke.h

@@ -65,6 +65,12 @@ typedef struct {
   unsigned char key[NKE_MAX_KEY_LENGTH];
 } NKE_Key;
 
+typedef struct {
+  SIV_Algorithm algorithm;
+  NKE_Key c2s;
+  NKE_Key s2c;
+} NKE_Context;
+
 typedef struct {
   int length;
   unsigned char cookie[NKE_MAX_COOKIE_LENGTH];