getPerfCounters() now uses scope tags

[Ev3nt]

The goal of this PR is to utilize new scope tags in isql. Now when using SET STATS we got this table of stats scopes:

Tag	Scope
isc_info_fetches	fb_info_counts_scope_att
isc_info_marks	fb_info_counts_scope_att
isc_info_reads	fb_info_counts_scope_att
isc_info_writes	fb_info_counts_scope_att
isc_info_current_memory	fb_info_counts_scope_db
isc_info_max_memory	fb_info_counts_scope_db

This change fixes the old issue with ISQL reporting unstable (depending on the concurrent load) page-level counters in the execution statistics.

[aafemt]

Not synchronized access to a global variable. Completely unnecessary additional round-trip. Not good, IMHO.

[dyemanov]

Completely unnecessary additional round-trip.

Do you suggest a try-except loop (first iteration with scopes, second without)?

[aafemt]

Do you suggest a try-except loop (first iteration with scopes, second without)?

Unknown info items don't throw, there is no need in try-catch. isc_info_error is already handled, so if server doesn't support scopes, they will be silently skipped by existing code and result will be exactly the same as before.

All that is needed is to add scope tags to the request buffer. They are cheap, so even optimization by grouping counters might be unnecessary, just fill the buffer with scope,tag pairs from the table.