add new pool handler based on dispcount

Like the default pool handler hackney_pool, but with the difference that
for each endpoint (domain/ip + port + ssl) of requests, a load balancer
is started allowing as many connections as mentioned in the
configuration.

Each load balancer has N workers that will connect on-demand to each
client.

The load balancer/pools/dispatcher mechanism is based on dispcount,
which will randomly contact workers. This means that even though few
connections might be required, the nondeterministic dispatching may make
all connections open at some point. As such, dlhttpc should be used in
cases where the load is somewhat predictable in terms of base levels.