Reset callbacks when Cartridge roles hot reload

[oleg-jukovec]

After #341 callbacks don't reset too. It seems like an unexpected behavior.

At least with cartridge, because each:

myapp.router> require("cartridge.roles").reload()
myapp.router> metrics = require("metrics")

adds 16 callbacks to:

metrics/metrics/registry.lua

Line 13 in b17a323

self.callbacks = {}

16, 32, 48...

Old callbacks may be outdated or use outdated data. If we call them, it can lead to unexpected errors.

[filonenko-mikhail]

Let's reset callbacks