sstables: optimize selection of sstables for leveled strategy
sstables: optimize selection of sstables for leveled strategy

It's possible to copy sstables directly into vector, and that will
improve performance. my benchmark tool[1] shows that new version
reduces running time of *copy procedure* by factor of two after
1024^2 calls.
Switching to back_inserter improves throughput even further.
[1]: gist.github.com/raphaelsc/a4b27290f362cdecdef399770dda759c

Refs #1632.

Signed-off-by: Raphael S. Carvalho <1e2942cea40e3b4d2162dfcd9e53e41c05a84d22@scylladb.com>
Message-Id: <7153514a9b5f5eb24dff518ee9fa3680e0881dae.1472741401.git.1e2942cea40e3b4d2162dfcd9e53e41c05a84d22@scylladb.com>

avikivity/scylla,scylladb/scylla,duarten/scylla,avikivity/scylla,avikivity/scylla,raphaelsc/scylla,duarten/scylla,scylladb/scylla,raphaelsc/scylla,raphaelsc/scylla,duarten/scylla,scylladb/scylla,scylladb/scylla