WL#2214: warnings should be reset only before the next batch of statements

Affects: Server-5.0   —   Status: Un-Assigned   —   Priority: Medium

Currently warnings are reset before each statement.  When executing a batch of
statements using the native server batch functionality, any warning generated by
statements before the last statement cannot be retrieve using SHOW WARNINGS.

[ Later pgulutzan comment ] On the other hand, the standard requires clearing
warnings before each statement.