log: introduce per-module log_level

[Gumix]

log: add log.new() function that creates a new logger
It allows to create a new instance of a log module, with a custom name, e.g.:

local my_log = require('log').new('my_module')

The name is added to the log message after fiber name:

YYYY-MM-DD hh:mm:ss.ms [PID]: CORD/FID/FIBERNAME/MODULENAME LEVEL> MSG

log: add per-module log level setting via log.cfg() or box.cfg()
Now it is possible to specify the log level for each module separately, e.g.:

box.cfg {
    log_level = 5,
    log_modules = {
        ['foo.bar'] = 1,
        expirationd = 'debug'
    }
}

The name of a module is determined automatically, or it is possible to create a logger with a custom name by using log.new().

log: implement automatic module name deduction
The name of a module is determined automatically during the execution of require('log') in the module's source code. The name is derived from its filename, including a part of the path. This is implemented by overriding the built-in require function.

Closes #3211

[locker]

I find it difficult to review this PR because of how it's split. I think it should be re-split as follows:

1. Add log.new() function that creates a new log module. The function should take the module name and the log level.
2. Add a utility module/function that can be usd to extract the module name from the file name or stack (the code that's currently located in src/lua/init.lua).
3. Implement automatic module name deduction in log.new() (something like log.new({module = 'auto'})).
4. Add new global log options.
   • Allow the user to enable automatic module name deduction via log.cfg().
   • Allow the user to set per-module log level via log.cfg() by passing a map{name->level} for opts.level.
5. Implement the box API mapping for the new log options.

Each part should be covered with tests.

We also need to agree on the names (the current API looks a bit awkward to me).

[coveralls]

Coverage increased (+0.02%) to 84.829% when pulling 9ba8167 on Gumix:iverbin/gh-3211-per-module-logging into 05002ed
on tarantool:master.

[Gumix]

Cartridge tests use some dirty magic, which doesn't work with require overloading:
https://github.com/tarantool/cartridge/blob/f241bf46f07b6bf2d5df38fba705720545a8218e/test/unit/upload_test.lua#L16-L18

Filed an issue: tarantool/cartridge#1998