Stasis: Use custom structure when setting variables.

A recent change to queue channel variable setting to the Stasis control
queue caused a regression. When setting channel variables, it is
possible to give a NULL channel variable value in order to unset the
variable (i.e. remove it from the channel variable list). The change
introduced a call to ast_variable_new(), which is not tolerant of NULL
channel variable values.

This new change switches from using ast_variable to using a custom
channel variable struct that is lighter weight and NULL value-tolerant.

Change-Id: I784d7beaaa3c036ea936d103e7caf0bb1562162d