Separate shared page limits for different purposes.

Separate shared page limits for different purposes.

The patch implements separate level counters for shared pages
with different purpose (i.e. memory cache and IPC I/O).  A new
purpose parameter was added to Ipc::Mem::GetPage() function to
update the level and check the limits.  After the change, memory
cache and IPC I/O limits are separate and do not affect each
other, i.e. full memory cache does not eat all IPC I/O pages.