thorium: restore the previous buffer size of 4 KiB
thorium: restore the previous buffer size of 4 KiB

The running time for important steps in Spate on Edison (Cray XC30) at
NERSC using 256 nodes are shown below.

4 KiB
TIMER [Build assembly graph / Distribute vertices] 3 minutes, 17.623886 seconds
TIMER [Build assembly graph / Distribute arcs] 6 minutes, 33.651123 seconds

8 KiB
TIMER [Build assembly graph / Distribute vertices] 5 minutes, 19.892883 seconds
TIMER [Build assembly graph / Distribute arcs] 10 minutes, 22.068970 seconds

Signed-off-by: Sébastien Boisvert <1a4d2e25eb5ba040b6ad5779a730a1a8a0ff9bc5@anl.gov>

sebhtml/biosal,GeneAssembly/biosal,sebhtml/biosal,sebhtml/biosal,GeneAssembly/biosal,sebhtml/biosal,GeneAssembly/biosal,GeneAssembly/biosal,sebhtml/biosal,GeneAssembly/biosal,GeneAssembly/biosal,sebhtml/biosal