Basic test suite for TCP drains

The send behaviour is documented, along with basic L10'ing.
There is otherwise not a need for tests as advanced as for the http
drain buffers given there is not a lot of similar error handling code
included. (TCP drains just ignore all send errors and keep going)
Bad shutdown fix

Part of tcpsyslog drains (when connected) were sensitive to a previous
race condition on shutdown
extend logplex_drain_buffer API

allow to resize the buffer dynamically, which would be useful to support
an API similar to the tcpsyslog one.
Remove stale code from http drains

Had to do with runtime compatibility in older versions, no longer
required.
Add prototype of tcpsylog drain with external buff

The prototype uses logplex_drain_buffer, which allows synchronous TCP
handling, allowing to get rid of raw prim_inet use, and simplifies the
state machine, on top of likely making the drain more resilient.
Fixing copyright line
Fixing bad shutdown in end_testcase

The end per testcase function is common to multiple tests, but some of
them do not use an active drain. This fix makes the end_per_testcase
function blind to whether it exists or not.
Basic benchmark for tcp drain implementations
Adding a way to unregister a channel

Tests are included. The feature will be required when upgrading
tcpsyslog drains from one kind to another -- the drain process itself
will need to transfer the channel property to the drain buffer process,
otherwise messages will be duplicated.
Adding upgrade procedure for tcpsyslog2

There is no direct update to be done between both components, but the
function could be used, for example, if logplex_tcpsyslog_drain decided
to use enter_loop to upgrade.
Replacing the old drains by the new one

Upgrade procedure left to be properly tested.
Fixing tests following merge of tcpsyslog drains

The benchmark is now skipped by default according to the spec,
and the drain suite swapped from logplex_tcpsyslog_drain2 back to
logplex_tcpsyslog_drain.
Upgrade path for v67->v68

A few stateless updates, and logic to upgrade the tcpsyslog drains
Fixing pids for upgrade script
Attempt to speed-up live upgrade

Let's go for concurrency since sequential stuff was so abysmally bad
on large deploys.
Supporting v67.1

Also adding better control for synchronous upgrading of the node.

Rolling the fleet might just be simpler than live upgrades at this
point.
Fix bad syntax in script
Requiring R16B
Partial support for cowboy 0.8.5

Also upgrades the header on bad responses to contain the explanation
instead of the generic HTTP message
Drop live code upgrade

Changes getting too complex, will require a rolling upgrade that tears
nodes down.