bshuf_block: some low-hanging-fruit optimizations on write path
bshuf_block: some low-hanging-fruit optimizations on write path

Rather than adding an element at a time and calling the virtual
'IsBlockFull()' function, we predetermine how many elements we will
accept. This allows a much simpler batched 'Add()' implementation.

This sped up the write side of cfile-test's 100M-integer test about 2x.

Change-Id: Ia895f7731e5371967782ef9cb176a9d493894a83
Reviewed-on: http://gerrit.cloudera.org:8080/5195
Reviewed-by: Dan Burkert <4ef91e292a55314d2653d35ff669a6bd939ce515@apache.org>
Tested-by: Kudu Jenkins

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