databases/cego: update 2.31.4 -> 2.31.5

• Changed CegoQueryHelper methods to static
• Fix for CegoBTreeManager and friends regarding small string values ( string(1) )
• Please note, that this fix invalidates btree format. Btrees have to be recreated after this fix, databases need to be exported and imported.

Submitted by: Bjoern Lemke <lemke@lemke-it.com>