Independent shared I/O page limit.

Independent shared I/O page limit.

Shared memory pages are used for shared memory cache and IPC I/O module.
Before this change, the number of shared memory pages needed for IPC I/O
was calculated from the size of shared memory cache.  Moreover, shared
memory cache was required for IPC I/O.

The patch makes the limit for shared I/O pages independent from the
shared memory cache size and presence.  IPC I/O pages limit is calculated
from the number of workers and diskers; it does not depend on cache_dir
configuration.  This may change in the future if we learn how to compute
it (e.g., by multiplying max-swap-rate and swap-timeout if both are
available).