Add btr pool support to the fecdec filter.

Add btr pool support to the fecdec filter.

The consumers of the output of fecdec are typically the decoders
which have requirements on the minimal amount of data available
in one chunk. Using a buffer pool minimizes the number of memcpy()
operations needed during btr_merge when the current buffer is too small.

To fit the needs of fecdec, a new function btr_copy() is introduced
which copies a data buffer into the btr pool area and generates
btr buffers and references. btr_add_output_pool() had to be adjusted
a bit and all callers have been changed to match.

The patch also adds a new field "name" to the buffer_pool structure.
This makes the log output more readable.