WIP on refreshing expired sessions

- Move RethinkDB operations to their own module
- Incorporated cancellation tokens throughout

src/RethinkDB.DistributedCache/CacheEntry.fs

@@ -0,0 +1,30 @@
+namespace RethinkDB.DistributedCache
+
+/// Persistence object for a cache entry
+[<CLIMutable; NoComparison; NoEquality>]
+type CacheEntry =
+    {   /// The ID for the cache entry
+        id : string
+        
+        /// The payload for the cache entry (as a UTF-8 string)
+        payload : string
+        
+        /// The ticks at which this entry expires
+        expiresAt : int64
+        
+        /// The number of ticks in the sliding expiration
+        slidingExp : int64
+        
+        /// The ticks for absolute expiration
+        absoluteExp : int64
+    }
+
+/// Field names for the above
+[<AutoOpen>]
+module private CacheEntry =
+    [<Literal>]
+    let expiresAt = "expiresAt"
+    [<Literal>]
+    let slidingExp = "slidingExp"
+    [<Literal>]
+    let absoluteExp = "absoluteExp"