features: Fix channel datastore access.

features: Fix channel datastore access.

Found as a result of the testsuite tests/callparking test crashing.

Several calls to ast_get_chan_featuremap_config() and
ast_get_chan_features_xfer_config() did not lock the channel before
calling so the channel's datastore list was accessed without the lock's
protection.  Apparently another thread deleted a datastore on the
channel's list while the crashing thread was walking the list.  Crash at
0xdeaddead due to MALLOC_DEBUG's memory filler value as a result.

* Add missing channel locks to calls that were not already protected
as the doxygen for those calls indicates.

Change-Id: Id273b3d305cc616406c353cbc841b2b7655efaa1