shard_connection: Support abstract Unix socket clients

[LINKIWI]

This PR introduces support for --unix-socket to accept an abstract Unix socket when the socket name prefixed with @.

Motivation: I am orchestrating execution of the memtier_benchmark binary against Redis protocol-compliant servers in an environment where the filesystem is not writable and binding TCP listeners is not preferred. Abstract Unix sockets are a convenient solution for this scenario.

Verification

Preparation

$ redis-server --port 6120

Unix socket file

$ socat -d UNIX-LISTEN:/tmp/redis.sock,reuseaddr,fork TCP:localhost:6120

$ netstat -ax | grep redis
unix  2      [ ACC ]     STREAM     LISTENING     223273   /tmp/redis.sock

$ ./memtier_benchmark --unix-socket /tmp/redis.sock --clients 1 --threads 1 --test-time 1
Writing results to stdout
[RUN #1] Preparing benchmark client...
[RUN #1] Launching threads now...
[RUN #1 100%,   1 secs]  0 threads:         173 ops,     171 (avg:     172) ops/sec, 7.31KB/sec (avg: 7.34KB/sec),  5.79 (avg:  5.78) msec latency

1         Threads
1         Connections per thread
1         Seconds


ALL STATS
============================================================================================================================
Type         Ops/sec     Hits/sec   Misses/sec    Avg. Latency     p50 Latency     p99 Latency   p99.9 Latency       KB/sec 
----------------------------------------------------------------------------------------------------------------------------
Sets           15.99          ---          ---         6.28750         3.32700        41.98300        41.98300         1.23 
Gets          156.88         0.00       156.88         5.72912         3.74300        22.27100        51.96700         6.11 
Waits           0.00          ---          ---             ---             ---             ---             ---          --- 
Totals        172.87         0.00       156.88         5.78076         3.61500        29.95100        51.96700         7.34

Abstract Unix socket

$ socat -d ABSTRACT-LISTEN:redis.sock,reuseaddr,fork TCP:localhost:6120

$ netstat -ax | grep redis
unix  2      [ ACC ]     STREAM     LISTENING     239347   @redis.sock

$ ./memtier_benchmark --unix-socket @redis.sock --clients 1 --threads 1 --test-time 1
Writing results to stdout
[RUN #1] Preparing benchmark client...
[RUN #1] Launching threads now...
[RUN #1 100%,   1 secs]  0 threads:         151 ops,     149 (avg:     150) ops/sec, 6.38KB/sec (avg: 6.41KB/sec),  6.64 (avg:  6.62) msec latency

1         Threads
1         Connections per thread
1         Seconds


ALL STATS
============================================================================================================================
Type         Ops/sec     Hits/sec   Misses/sec    Avg. Latency     p50 Latency     p99 Latency   p99.9 Latency       KB/sec 
----------------------------------------------------------------------------------------------------------------------------
Sets           13.99          ---          ---        12.56800         4.41500       111.61500       111.61500         1.07 
Gets          136.92         0.00       136.92         6.01606         4.04700        29.69500        38.91100         5.34 
Waits           0.00          ---          ---             ---             ---             ---             ---          --- 
Totals        150.91         0.00       136.92         6.62352         4.15900        29.69500       111.61500         6.41

[LINKIWI]

@filipecosta90 This is still a patch we are interested in. Could you help review this? Thanks.