lua/fiber: add max slice to fiber info

This commit adds the new field 'max_slice' to fiber.info() that reports
the max slice applied to the given fiber. The value is a table with two
fields: 'err' for error slice and 'warn' for warning slice. Values
greater than or equal to TIMEOUT_INFINITY aren't reported.

Follow-up #6085

NO_DOC=tarantool/doc#3057
NO_CHANGELOG=unreleased