Support handling TCP I/O in simple separate io_uring-based I/O threads

Required mainly for clients, allows to scale parallel client I/O with TCP
from 100-150k iops to ~400k iops and from 2-3 GB/s to at least 7-8 GB/s
with 4 I/O threads, at the same time increasing Q=1 latency by 2x thread
switching delay, which is ~10 us when CPU powersaving is disabled and may
be as high as 200 us when it's enabled.

docs/config/client.en.md

@@ -9,6 +9,7 @@
 These parameters apply only to Vitastor clients (QEMU, fio, NBD and so on) and
 affect their interaction with the cluster.
 
+- [client_iothread_count](#client_iothread_count)
 - [client_retry_interval](#client_retry_interval)
 - [client_eio_retry_interval](#client_eio_retry_interval)
 - [client_retry_enospc](#client_retry_enospc)
@@ -23,6 +24,23 @@ affect their interaction with the cluster.
 - [nbd_max_part](#nbd_max_part)
 - [osd_nearfull_ratio](#osd_nearfull_ratio)
 
+## client_iothread_count
+
+- Type: integer
+- Default: 0
+
+Number of separate threads for handling TCP network I/O at client library
+side. Enabling 4 threads usually allows to increase peak performance of each
+client from approx. 2-3 to 7-8 GByte/s linear read/write and from approx.
+100-150 to 400 thousand iops, but at the same time it increases latency.
+Latency increase depends on CPU: with CPU power saving disabled latency
+only increases by ~10 us (equivalent to Q=1 iops decrease from 10500 to 9500),
+with CPU power saving enabled it may be as high as 500 us (equivalent to Q=1
+iops decrease from 2000 to 1000). RDMA isn't affected by this option.
+
+It's recommended to enable client I/O threads if you don't use RDMA and want
+to increase peak client performance.
+
 ## client_retry_interval
 
 - Type: milliseconds