SNOW-914532: Use async for session pool finalizer (#790)

### Description
Regarding issue 648

The PR fixes a hanging issue due to a [possible
deadlock](https://github.com/snowflakedb/snowflake-connector-net/blob/127343fb4f8a55597df3bf3f20e64e18975fcb62/Snowflake.Data/Core/RestRequester.cs#L54)
by using async instead when the finalizer tries to close all the
sessions in the pool

### Checklist
- [x] Code compiles correctly
- [x] Code is formatted according to [Coding
Conventions](../CodingConventions.md)
- [x] Created tests which fail without the change (if possible)
- [x] All tests passing (`dotnet test`)
- [x] Extended the README / documentation, if necessary
- [x] Provide JIRA issue id (if possible) or GitHub issue id in PR name

Snowflake.Data/Core/Session/SessionPool.cs

@@ -36,7 +36,9 @@ internal SessionPool()
 
         ~SessionPool()
         {
-            ClearAllPools();
+            // Use async for the finalizer due to possible deadlock
+            // when waiting for the CloseResponse task while closing the session
+            ClearAllPoolsAsync();
         }
 
         public void Dispose()
@@ -191,6 +193,16 @@ internal void ClearAllPools()
             }
         }
 
+        internal async void ClearAllPoolsAsync()
+        {
+            s_logger.Debug("SessionPool::ClearAllPoolsAsync");
+            foreach (SFSession session in _sessionPool)
+            {
+                await session.CloseAsync(CancellationToken.None).ConfigureAwait(false);
+            }
+            _sessionPool.Clear();
+        }
+
         public void SetMaxPoolSize(int size)
         {
             _maxPoolSize = size;