Sink.Close() working again

Fix side effects of sink.Close() refs #4 and #5

Fixed in a straight way by removing logic with sink ID. The logic was
corrupted any way. Aman Gupta explained the problem in #5 and figured
out the cause with sink ID. In this change I suppose that a number of
sinks will not very large.

Use slice in the context instead of maps

The context uses now slices for storage instead of maps. I found that I have no source code with the number of items in the context greater than ten. With such number of items the search in the slice is more efficient than in map. Also, the usage of the slice fixes order of context records that was not determined with maps.