Various bug fixes and improvements for the market

[devos50]

In this PR, I fixed some bugs in the market community. I also removed a log statement in the tunnel community that slowed anonymous downloads. I decided to have the improvement as suggested by @qstokkink in a separate PR since it did not work out of the box and this PR was growing quite a lot with fixes/improvements for the market community.

I also upgraded the Electrum wallet to 2.9.4 and tested some basic trading functionality between two different machines, with Testnet Bitcoin wallets.

Working market experiment with this PR: https://jenkins.tribler.org/job/pers/job/market_experiment_devos50/745/

Fixes #3494

[qstokkink]

Please also include this in this PR:

Make the session dispatching more efficient here:
https://github.com/devos50/tribler/blob/777f59a08c09131fc94760f4b03bb060395799a7/Tribler/community/triblertunnel/dispatcher.py#L44
Through a map of the circuit_id to udp_connection.socksconnection (which is removed on circuit_dead) here:
https://github.com/devos50/tribler/blob/777f59a08c09131fc94760f4b03bb060395799a7/Tribler/community/triblertunnel/dispatcher.py#L57

[qstokkink]

ERROR   1521022296  test_as_server:TestTunnelCommunity:124  The reactor was dirty during tearDown:
ERROR   1521022296  test_as_server:TestTunnelCommunity:126  >     <DelayedCall 0x7f065a166758 [7.30270910263s] called=0 cancelled=0 RequestCache._on_timeout(<Tribler.pyipv8.ipv8.messaging.anonymization.caches.CreatedRequestCache object at 0x7f065a154f90>)>

[xoriole]

Please update the docstring to replace get request with post

[qstokkink]

Please use https://github.com/Tribler/tribler/blob/next/Tribler/Core/Utilities/json_util.py

[qstokkink]

Apparently the test is failing on this call, which is leaking a DelayedCall:
https://github.com/Tribler/py-ipv8/blob/master/ipv8/requestcache.py#L96

self.register_task(cache, reactor.callLater(cache.timeout_delay, self._on_timeout, cache))

This might have to be a deferLater instead.