Log can be flooded by warning messages

[locker]

For instance, when tarantool hits the limit on open files, the log is flooded with thousands messages like below, which makes it difficult to read the log.

2017-03-18 10:09:40.788 [19872] iproto sio.cc:251 !> SystemError accept, called on fd 13, aka 0.0.0.0:3301: Too many open files
2017-03-18 10:09:40.788 [19872] iproto sio.cc:251 !> SystemError accept, called on fd 13, aka 0.0.0.0:3301: Too many open files
2017-03-18 10:09:40.788 [19872] iproto sio.cc:251 !> SystemError accept, called on fd 13, aka 0.0.0.0:3301: Too many open files
2017-03-18 10:09:40.788 [19872] iproto sio.cc:251 !> SystemError accept, called on fd 13, aka 0.0.0.0:3301: Too many open files
2017-03-18 10:09:40.788 [19872] iproto sio.cc:251 !> SystemError accept, called on fd 13, aka 0.0.0.0:3301: Too many open files
2017-03-18 10:09:40.788 [19872] iproto sio.cc:251 !> SystemError accept, called on fd 13, aka 0.0.0.0:3301: Too many open files

We need to come up with a way of limiting the rate at which messages can be printed to the log.

[Igorjan94]

which makes it difficult to read the log

Fro me, when tarantool starts "spam" with these messages, CPU becomes 100% and tarantool refuses to response at all

[locker]

To fix this issue, we need to introduce say_ratelimited(), which takes a rate limit state as an additional argument and suppresses the message if the rate limit is exceeded, similarly to how it is done here. This way, for each particular say() call site, we can decide whether it needs rate limiting or not and if it does how many messages can be printed per unit of time tops.

SocketError, which is the source of log spamming in this issue, should have a rate limit state shared among all objects of this class. The class then should override Exception::log() method so that it uses say_ratelimited() with the shared rate limit state instead of say(). This should resolve the log spamming problem described here and lay the foundation for fixing similar issues that may be reported in future.