master: Add I/O limits configuration

Rename IoLimitConfigLoader to IoLimitsConfigLoader (plural Limits).

Move IoLimitsConfigLoader to common.

Load globaliolimits.cfg and other I/O limiting options in master.
master: Add IoLimitsDatabase

Add a class which stores information about global I/O limits.
master: Support registration of I/O limited mounts

Detect clients that support I/O limiting and enable it for them.
master: Support I/O limit negotiations

Allow clients to negotiate changes of I/O limits.