ECSubWrite: pass some parameter by reference

CID 1188143 (#1 of 1): Big parameter passed by value (PASS_BY_VALUE)
 pass_by_value: Passing parameter stats of type pg_stat_t (size 488 bytes)
  by value.
 pass_by_value: Passing parameter t of type ObjectStore::Transaction (size
  184 bytes) by value.

Signed-off-by: Danny Al-Gaaf <danny.al-gaaf@bisect.de>