rpcban fixes

[jonasschnelli]

• fix a missing lock in clearban
• add Ipv6 tests

[laanwj]

Travis error looks very serious, and occurs in the node handling test:

*** buffer overflow detected ***: /home/travis/build/bitcoin/bitcoin/bitcoin-x86_64-unknown-linux-gnu/src/bitcoind terminated
======= Backtrace: =========
/lib/x86_64-linux-gnu/libc.so.6(__fortify_fail+0x37)[0x7faed05b9007]
/lib/x86_64-linux-gnu/libc.so.6(+0x107f00)[0x7faed05b7f00]
/home/travis/build/bitcoin/bitcoin/bitcoin-x86_64-unknown-linux-gnu/src/bitcoind(__fdelt_chk+0x22)[0x7faed1ce2d25]
/home/travis/build/bitcoin/bitcoin/bitcoin-x86_64-unknown-linux-gnu/src/bitcoind(+0x2315e3)[0x7faed1b115e3]
/home/travis/build/bitcoin/bitcoin/bitcoin-x86_64-unknown-linux-gnu/src/bitcoind(_Z11TraceThreadIPFvvEEvPKcT_+0x78)[0x7faed1b33f18]
/home/travis/build/bitcoin/bitcoin/bitcoin-x86_64-unknown-linux-gnu/src/bitcoind(+0x235adb)[0x7faed1b15adb]
/home/travis/build/bitcoin/bitcoin/bitcoin-x86_64-unknown-linux-gnu/src/bitcoind(+0x568294)[0x7faed1e48294]
/lib/x86_64-linux-gnu/libpthread.so.0(+0x7e9a)[0x7faed0877e9a]
/lib/x86_64-linux-gnu/libc.so.6(clone+0x6d)[0x7faed05a24bd]
===== Memory map: ========

[jonasschnelli]

Oh. Right. This is serious. I'm investigating...

[jonasschnelli]

Added a commit that fixes a race condition during node disconnecting.
Before this PR, we where using CNode::CloseSocketDisconnect() to disconnect nodes from the UI's peer table context menu as well over the rpc ban and disconnectnode functions. CNode::CloseSocketDisconnect() does close the socket directly, which is not thread safe because of missing locks around the socket handler.

This PR changes the disconnecting to go over the CNode::fDisconnect flag (which causes to disconnect the node during the existing node threads).

[laanwj]

Tested ACK.
I would call this "fix" not "overhaul".

[jonasschnelli]

Agreed. Changed PR title.