Added OutputChannel::propagateAsDecision() functionality, allowing a theory

Added OutputChannel::propagateAsDecision() functionality, allowing a theory
to request a decision on a literal.  All these theory requests are kept in a
context-dependent queue and serviced in order when the SAT solver goes to make a
decision.  Requests that don't have a SAT literal give an assert-fail.  Requests
for literals that already have an assignment are silently ignored.

Since the queue is CD, requests can actually be serviced more than once (e.g., if
a request is made at DL 5, but not serviced until DL 10, and later, a conflict
backtracks to level 7, the request may be serviced again).

Performance impact: none to negligible for theories that don't use it
  See http://church.cims.nyu.edu/regress-results/compare_jobs.php?job_id=3620&reference_id=3614&mode=&category=&p=0