Add http pool to reduce connection overhead

src/http_pool.nim

@@ -0,0 +1,37 @@
+import asyncdispatch, httpclient
+
+type
+  HttpPool* = ref object
+    conns*: seq[AsyncHttpClient]
+
+var maxConns {.threadvar.}: int
+
+let keepAlive* = newHttpHeaders({
+  "Connection": "Keep-Alive"
+})
+
+proc setMaxHttpConns*(n: int) =
+  maxConns = n
+
+proc release*(pool: HttpPool; client: AsyncHttpClient) =
+  if pool.conns.len >= maxConns:
+    client.close()
+  elif client != nil:
+    pool.conns.insert(client)
+
+template use*(pool: HttpPool; heads: HttpHeaders; body: untyped): untyped =
+  var c {.inject.}: AsyncHttpClient
+
+  if pool.conns.len == 0:
+    c = newAsyncHttpClient(headers=heads)
+  else:
+    c = pool.conns.pop()
+    c.headers = heads
+
+  try:
+    body
+  except ProtocolError:
+    # Twitter closed the connection, retry
+    body
+  finally:
+    pool.release(c)