Speed up scan

[badrishc]

Avoid buffer pool allocation per GetNext and reuse memory to speed up scan.
Avoid double counting for dbsize.

Fixes #858

[badrishc]

Command:
time memtier_benchmark -s 192.168.192.1 -p6379 -Predis --hide-histogram --command="SCAN 0 MATCH * COUNT 1000" -c 10 -t 4

main:

==================================================================================================
Type         Ops/sec    Avg. Latency     p50 Latency     p99 Latency   p99.9 Latency       KB/sec
--------------------------------------------------------------------------------------------------
Scans        1245.57        32.33031        33.40700        69.50300        81.79100     15778.88
Totals       1245.57        32.33031        33.40700        69.50300        81.79100     31557.77

real    5m25.291s
user    1m2.464s
sys     0m9.081s

PR:

==================================================================================================
Type         Ops/sec    Avg. Latency     p50 Latency     p99 Latency   p99.9 Latency       KB/sec
--------------------------------------------------------------------------------------------------
Scans       21843.85         1.76293         1.55100         4.78300        14.59900    276717.20
Totals      21843.85         1.76293         1.55100         4.78300        14.59900    553434.40

real    0m19.247s
user    0m46.319s
sys     0m10.980s