Make log level configurable

[kuzmig]

Currently, log levels of SIP stack, transport and transaction layers are set to debug.
There is a way to change SIP stack log level but there is no way to change log level of transport and transaction layers.
Please, make it possible to configure it. It may also make sense to set log level to info by default.

[jspc]

I have a similar request here- I personally don't want my dependencies logging anything I haven't asked them explicitly to. I'd love a way of disabling these unless I ask