Changes

register only a single poll_t with libuv

Most of the time we only need a read _or_ a write callback registered
with libuv - for example, on a UDP request a write callback is
registered, when executed the write callback performs the write,
deregisters itself, and registers a read callback.

However there is one case where getdns registers both read and write
callbacks: when a backlog of TCP requests is going to the same upstream
resolver, we use a single fd and queue the requests. In this instance we
want to listen for both read (to get responses for requests we've
already sent) and write (to continue to send our pending requests).

libuv, like most event libraries, only allows one callback to be
registered per fd. To get notification for both reads and writes, you
should examine the event flags and have appropriate conditional logic
within the single callback. Today getdns incorrectly tries to register
two separate poll_t with libuv, one for read and one for write - this
results in a crash (internal libuv assertion guaranteeing that only a
single poll_t is registered per fd).

Testing was done by using flamethrower
(https://github.com/DNS-OARC/flamethrower) to toss queries at a program
that embeds getdns.

Note that a higher qps trigger a _different_ getdns/libuv crashing bug
that occurs when the TCP backlog grows so large that requests start to
time out. That crash is not addressed in this PR, and will be more
involved to fix.
 (commit: 2673a57)