FlowControl: credits are unserialized incorrectly

This is a regression in 3.6.9 (3.6.8 is fine). In FlowControl.sendCredits() and .sendCreditRequest(), we use Util.objectToBuffer() to serialize the credits, but on the receiver side we use Bits.readLong(), which interprets the TYPE_LONG sent as first byte as being part of the long.

The reason is an imcomplete backport of from the master branch. The problem does not exist on master.

The faulty commit is ac5f507 on the 3.6 branch