libsframe: rename encoder to ectx for readability

Addressing (an old) review comment suggesting this housekeeping item.
Use consistent naming style in libsframe.  sframe_decoder_ctx objects
are named 'dctx', so use 'ectx' for sframe_encoder_ctx objects.

Make necessary changes in all the applicable declarations and definitions.

Reviewed-by: Jens Remus <jremus@linux.ibm.com>