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* Share common code between network/ipfs_impl_test.go tests

Extract the code that is common in TestMessageResendAfterError,
TestMessageSendTimeout and TestMessageSendNotSupportedResponse to a
separate function.

* Make prepareNetwork() return two hosts and two networks

Let prepareNetwork() make simmetric setup with two `ErrHost`s
with two `impl` networks to be sure we test `impl` instances on
both ends.

* Added TestNetworkCounters test to the "network" package

The test shows we have a problem with `MessagesSent` counter.

* Fix: Increment stats.MessagesSent in msgToStream() function

Fixes the bug with incrementing `MessagesSent` counter only in
`SendMessage()` method if `impl`. Now it works for `MessageSender`
too.

* Allow to specify a network event listener for tests

Added `listener network.Notifiee` to the `receiver` structure.
If a listener is specified then `prepareNetwork()` connects it
to the mock network it builds before making any connections.

* Wait for all network streams are closed in testNetworkCounters

Wait for all network streams are closed instead of just using a
timeout. The timeout of 5 s is still used as a deadline (it makes
the test to fail).

* Fix: Close the MessageSender in testNetworkCounters()

The `MessageSender` needs to be closed if we want all streams in
the network to be closed.

* Fix: Close MessageSender in other tests too
Co-authored-by: Paul Wolneykien <manowar@altlinux.org>

We use these outside of bitswap for testing.

This makes reading the docs much easier as it's clear what's "private" and
what's not.

fixes #238

This commit extends the bitswap protocol with two additional wantlist properties:

* WANT_HAVE/HAVE: Instead of asking for a block, a node can specify that they
  want to know if any peers "have" the block.
* WANT_HAVE_NOT/HAVE_NOT: Instead of waiting for a timeout, a node can explicitly
  request to be told immediately if their peers don't currently have the given
  block.

Additionally, nodes now tell their peers how much data they have queued to send
them when sending messages. This allows peers to better distribute requests,
keeping all peers busy but not overloaded.

Changes in this PR are described in: https://github.com/ipfs/go-bitswap/issues/186

delay binding of network until a receiver is present. also add test of ipfs host network