TBX: x.x: optimize TXM

TXM must not exceed machine ressources limits (memory, etc.) when it is not necessary for a given process (aka ‘export concordance syndrom’…).
TXM architecture should cope with big corpora: be more efficient or degrade/curb a particular functionality to suggest an acceptable response

(from redmine: issue id 764, created on 2014/04/25 by Matthieu Decorde)