Provide ability to see how many buffers are used by some attachment when FB running in SC/CS mode

[pavel-zotov]

It will be good to see how many page buffers is used by attachment when it established connection with specifying that number (e.g. "isql -cache 12345"). Of course, this is actual only for CS /SC mode.

For example, if two users establish attachments in following way:

C:\FB\50CS>isql /:employee -user vasya -pas 123 -cache 1234
C:\FB\50CS>isql /:employee -user fedya -pas 456 -cache 2345

-- then DBA has no ability to see their memory consumption because mon$page_buffers present only in mon$tadatabase.