input_seqsrc: update the counters on input->stream immediately

previously the seqsrc tracking code would only update
input->stream->count_drop and input->stream->count_recv when individual
seqsrc objects were garbage collected in _input_seqsrc_get(), or when
seqsrc tracking for an entire stream was shut down in
_input_seqsrc_destroy().

we want to be able to export the drop and recv counters for an entire
input stream on demand and for these counters to be accurate, so
increase the input stream counters immediately.
input: don't modify input->stream counters from seqsrc internals

ideally the input->stream->count_recv counter should be retrievable
without seqsrc tracking enabled, so increment count_recv in
_input_nmsg_read_container_sock() instead.

also, it's not necessary for the seqsrc tracking code to be responsible
for updating input->stream->count_drop. instead, return the number of
dropped containers from _input_seqsrc_update() and do the update of
input->stream->count_recv from _input_nmsg_read_container_sock().  this
keeps all the input->stream counter updates in the same function.