cdr_pgsql, cel_pgsql: Store maximum buffer size to prevent reallocation

The code previously used a fixed size of 512 for the SQL
queries. Depending on the size this may require it to grow.

This change makes it so if the buffer size does grow the size
is stored and next time the buffer will be large enough.

Change-Id: I55385899f1c06dee47e4274c2d21538037b2d895