Test client will only start with flag set to True.
Change EventsGenericClientTestCase to set the flag on the
first line of setUp.

The events tests check for register and emit signals,
but because the flag set_events_enabled was False by
default in the tests, no signals were being emitted.

I added the flag to the setUp and tearDown of the tests,
they are still very slow but at least they are passing now

Two test cases were broken and were implemented here:

The first was that HTTPClient should share the connection between
clients if a pool was not passed explicitly. If you initialize an
HTTPClient without a pool, it will reuse a pool created on the class.

The second was that you should be able to pass to the HTTPCLient a pool
on initialization. Added that possibility and fixed the tests accordingly

The events ioloop client is not completelly thread safe, so we have to use
reactor.callFromThread whenever we need it to be thread-safe. Examples of this
behaviour are the events ioloop client tests that depend on the firing of
deferreds, which are not thread safe. This commit fixes tese tests.

As @meskio pointed out, some cases could need a dedicated pool with
different parameters.
This is a suggested implementation where the pool is reused by default,
creating a dedicated one just if needed/asked. This way we ensure that
resources are under control and special cases are still handled.

Before this commit, protobuf and protobuf.socketrpc were used to serialize and
transmit messages between events clients. This change implements a simpler ZMQ
client/server events mechanism that uses ZMQ sockets for transmitting messages
from clients to server and to redistribute such messages to subscribed
clients.

Closes: #6359