gattc: Extend gattc queue with mtu exchange

Patch allows to exchange mtu using queue. Without this patch there was
no possiblity to exchange mtu mixing this synchronous and queue api.

Tag: #feature
Test: CtsVerifier
Sponsor: jpawlowski@
Change-Id: I2dd699d78de1b9eee8c683969f0c6586ccd21780