Use Unix socket for proxy client <-> proxy IPC

[redlicha]

Currently a TCP socket over loopback or another local interface is used. This requires requests / responses to traverse the TCP/IP stack and that recv completions are handled in softirq context. Unix sockets allow to avoid that, which shows better performance / less CPU consumption in microbenchmarks [1].

Using the "abstract namespace" feature offered by Linux, Unix sockets can be used transparently, i.e. without putting sockets on the local filesystem / requiring configuration for this.

[1] a voldrv test that either uses a Unix socket (0) or a TCP socket over localhost (1) on a 4 x i5-6440HQ CPU @ 2.60GHz :

jenkins@9284e706203e:~$ LOCOREM_CLIENTS=8 LOCOREM_MESSAGE_COUNT=100000 LOCOREM_HEADER_SIZE=4200 LOCOREM_SERVER_THREADS=1 ./youtils_test --gtest_filter=*LocORem*echo* --disable-logging
Note: Google Test filter = *LocORem*echo*
[==========] Running 2 tests from 1 test case.
[----------] Global test environment set-up.
[----------] 2 tests from LocORemTests/LocORemTest
[ RUN      ] LocORemTests/LocORemTest.echo/0
100000 iterations with headers of 4200 and messages of 0 bytes, 8 clients took 5.63182 seconds -> 142050 IOPS
[       OK ] LocORemTests/LocORemTest.echo/0 (5632 ms)
[ RUN      ] LocORemTests/LocORemTest.echo/1
100000 iterations with headers of 4200 and messages of 0 bytes, 8 clients took 7.66188 seconds -> 104413 IOPS
[       OK ] LocORemTests/LocORemTest.echo/1 (7662 ms)
[----------] 2 tests from LocORemTests/LocORemTest (13294 ms total)

[----------] Global test environment tear-down
[==========] 2 tests from 1 test case ran. (13294 ms total)
[  PASSED  ] 2 tests.

[domsj]

Why is this urgent/SRP ?
The proxy is no longer in the hot path, so imo this performance optimisation isn't urgent/important.