Support connections over unix sockets

[telepenin]

Hello, this PR provided support connections over unix sockets to nsqd.

According PR to nsq

[telepenin]

Tests are provided, but now it will failure on the github workflow due to unable tests against master branch of nsq.

[mreiferson]

Gonna leave this one for @ploxiln since you've been working with him on nsqio/nsq#1434

[ploxiln]

please do not change the 4-space indent to 2-space indent in the shells scripts

(4 space indent is the style we use for shell, and also it unnecessarily increases the diff to review)

[ploxiln]

there are some test failures:

 === RUN   TestUnixSocketProducerConnection
    producer_unix_socket_test.go:24: should lazily connect - dial tcp: address /tmp/nsqd.sock: missing port in address
--- FAIL: TestUnixSocketProducerConnection (0.00s)
=== RUN   TestUnixSocketProducerPing
    producer_unix_socket_test.go:46: should connect on ping
--- FAIL: TestUnixSocketProducerPing (0.00s)
=== RUN   TestUnixSocketProducerPublish
    producer_unix_socket_test.go:69: error dial tcp: address /tmp/nsqd.sock: missing port in address
--- FAIL: TestUnixSocketProducerPublish (0.00s)

[ploxiln]

... also I think we may need to add an un-released version of nsqd to the test matrix, and mask the new tests with a build tag that we enable only when testing against the newest build of nsqd.