planner: record the reason why a query can't hit the plan cache in statements_summary table

[qw4990]

Enhancement

User Story: In PoC, after a round of testing, the user wants to know which queries cannot hit the plan cache and their reasons to help them optimize the plan cache hit ratio further, but our current diagnosis information is not enough for this.

A graceful and simple method to implement this is to record the un-cacheable reason in the statements_summary table, then the user can query this table to identify the reason, for example:

select query_text, exec_count, uncacheable_reason 
from statements_summary 
where prepared=1 and uncacheable_reason!="" 
order by exec_count limit 100;