Deprecate and obsolete max_binlog_files with max_binlog_space

Migrated to https://jira.percona.com/browse/PS-275

The max_binlog_files feature wasn't thought through well enough and has
a couple of important drawbacks:

- it is inconsistent with a similar variable for relay logs
  (--relay-log-space-limit). It's reasonable to implement a similar
  variable for binary logs with the same semantics.

- max_binlog_files is hard (and sometimes dangerous) to use in practice,
  because server restarts and FLUSH LOGS make the server start new
  files, rendering the variable useless from the user perspective.

We should deprecate max_binlog_files and introduce max_binlog_space
(binlog_space_limit?) that would obsolete max_binlog_files.