mount: move and rename IoLimitConfigLoader

Rename IoLimitConfigLoader to IoLimitsConfigLoader (plural "Limits").

Move IoLimitsConfigLoader from mount to common.
master: Add I/O limits configuration

Use IoLimitConfigLoader to load globaliolimits.cfg.

Load other I/O limiting options from mfsmaster.cfg.
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 register them in
appropriate data structures.
master: Support I/O limit negotiations

Allow clients to negotiate changes in their I/O limits.