Reduce system calls of write for client->reply by introducing writev (#9934)

There are scenarios where it results in many small objects in the reply list,
such as commands heavily using deferred array replies (`addReplyDeferredLen`).
E.g. what COMMAND command and CLUSTER SLOTS used to do (see #10056, #7123),
but also in case of a transaction or a pipeline of commands that use just one differed array reply.

We used to have to run multiple loops along with multiple calls to `write()` to send data back to
peer based on the current code, but by means of `writev()`, we can gather those scattered
objects in reply list and include the static reply buffer as well, then send it by one system call,
that ought to achieve higher performance.

In the case of TLS,  we simply check and concatenate buffers into one big buffer and send it
away by one call to `connTLSWrite()`, if the amount of all buffers exceeds `NET_MAX_WRITES_PER_EVENT`,
then invoke `connTLSWrite()` multiple times to avoid a huge massive of memory copies.

Note that aside of reducing system calls, this change will also reduce the amount of
small TCP packets sent.