Sync scan & regression tests

[Konstantin Knizhnik]

Hi hackers,

Is it is ok, that regression tests do not pass with small value of 
shared buffers (for example 1Mb)?

Two tests are failed because of sync scan - this tests cluster.sql and 
portals.sql perform seqscan without explicit order by and expect that 
data will be returned in particular order. But because of sync scan it 
doesn't happen. Small shared buffers are needed to satisfy seqscan 
criteria in heapam.c: `scan->rs_nblocks > NBuffers / 4` for tenk1 table.

More general question - is it really good that in situation when there 
is actually no concurrent queries, seqscan is started not from the first 
page?