Add FreeBSD AIO support.

[vitlibar]

I hereby agree to the terms of the CLA available at: https://yandex.ru/legal/cla/?lang=en

#4302

Category (leave one):

• Improvement

Short description (up to few sentences):
Implemented AIO support for FreeBSD.

[proller]

../dbms/src/IO/AIO.cpp: In function 'int io_getevents(int, long int, long int, kevent*, timespec*)':
../dbms/src/IO/AIO.cpp:123:32: warning: parameter 'min_nr' set but not used [-Wunused-but-set-parameter]
 int io_getevents(int ctx, long min_nr, long max_nr, struct kevent * events, struct timespec * timeout)
                           ~~~~~^~~~~~
../dbms/src/IO/AIO.cpp: In constructor 'AIOContext::AIOContext(unsigned int)':
../dbms/src/IO/AIO.cpp:130:37: warning: parameter 'nr_events' set but not used [-Wunused-but-set-parameter]
 AIOContext::AIOContext(unsigned int nr_events)
                        ~~~~~~~~~~~~~^~~~~~~~~

[proller]

../dbms/src/IO/WriteBufferAIO.cpp:202:32: warning: use of old-style cast [-Wold-style-cast]
    bytes_written = aio_return((struct aiocb *)event.udata);
                               ^               ~~~~~~~~~~~
1 warning generated.
[1093/1771] Building CXX object dbms/CMakeFiles/clickhouse_common_io.dir/src/IO/ReadBufferAIO.cpp.o
[1094/1771] Building CXX object dbms/CMakeFiles/clickhouse_common_io.dir/src/IO/AIOContextPool.cpp.o
../dbms/src/IO/AIOContextPool.cpp:98:41: warning: use of old-style cast [-Wold-style-cast]
        it->second.set_value(aio_return((struct aiocb *)event.udata));
                                        ^               ~~~~~~~~~~~
1 warning generated.

[proller]

../dbms/src/IO/AIO.cpp:115:6: warning: variable 'r' may be uninitialized when used here [-Wconditional-uninitialized]
        if (r < 0) {
            ^
../dbms/src/IO/AIO.cpp:93:10: note: initialize the variable 'r' to silence this warning
    int r;
         ^
          = 0

../dbms/src/IO/AIOContextPool.cpp:98:41: warning: use of old-style cast [-Wold-style-cast]
        it->second.set_value(aio_return((struct aiocb *)event.udata));
                                        ^               ~~~~~~~~~~~

../dbms/src/IO/WriteBufferAIO.cpp:202:32: warning: use of old-style cast [-Wold-style-cast]
    bytes_written = aio_return((struct aiocb *)event.udata);
                               ^               ~~~~~~~~~~~

[alexey-milovidov]

Missing changelog.
Added.