Address warning reported by thread safety analyzer

Motivation:

Thread safety analyzer reports warning about observable state
leaving lock guarded code blocks. This PR address the warnings.

Analysis of the warning does not prove that we have a real bug and all the
warnings are considered as potential bugs.

Analyzer used is:
`docker run --rm -v $(pwd):/src -w /src swift:5.3.1 swift test -c
release --sanitize=thread --enable-test-discovery`

Modifications:

* accessor to count of connections in connection pool is guarded by lock
* forced materialization of a potentially lazy collection into an array before returning
  the collection from lock guarded code block

Result:

Most of thread safety warnings are addressed without modification of observable
code behaviour.

Author: ya-pulser

Diff for: Sources/AsyncHTTPClient/ConnectionPool.swift

@@ -103,14 +103,19 @@ final class ConnectionPool {
 
     func close(on eventLoop: EventLoop) -> EventLoopFuture<Bool> {
         let providers = self.lock.withLock {
-            self.providers.values
+            // we materialize Values into an Array before returning from under a lock
+            // as thread safety analyzer considers safety of Values as undefined and dependant on implementation
+            // e.g. a lazy collection implementation could by thread unsafe to be returned from within a lock
+            Array(self.providers.values)
         }
 
         return EventLoopFuture.reduce(true, providers.map { $0.close() }, on: eventLoop) { $0 && $1 }
     }
 
     var count: Int {
-        return self.providers.count
+        return self.lock.withLock {
+            return self.providers.count
+        }
     }
 
     /// Used by the `ConnectionPool` to index its `HTTP1ConnectionProvider`s