WL#11345: Increase default value of optimizer_trace_max_mem_size to a reasonable size

optimizer_trace_max_mem_size: The maximum cumulative size of stored optimizer
traces. 

The default value of optimizer_trace_max_mem_size is currently 16 kB, which
means that it is truncated for any non-trivial query. This causes a lot of pain
in the communication between development team and support, because many users
are unaware that their optimizer traces are indeed truncated. The default value
should be bumped to a larger value that accommodates most generated optimizer
traces.

Suggested new default value: 1 MB.

F1. Increase default value of optimizer_trace_max_mem_size from 16 kB to 1 MB.

There are no non-functional requirements.