Optimization: use write index cache in pop

[bvbfan]

Running benchmark on my machine:
without patch

SPSCQueue:
27176 ops/ms
193 ns RTT
boost::lockfree::spsc:
18873 ops/ms
298 ns RTT

with patch:

SPSCQueue:
41091 ops/ms
189 ns RTT
boost::lockfree::spsc:
18207 ops/ms
290 ns RTT

[bugacha]

@bvbfan did you compare your implementation with https://github.com/drogalis/SPSC-Queue ?

I'm curious how fast it is against yours.

[bvbfan]

@bvbfan did you compare your implementation with https://github.com/drogalis/SPSC-Queue ?

I'm curious how fast it is against yours.

dro::SPSCQueue: 
Mean: 23289 ops/ms 
Median: 23037 ops/ms 
Mean: 227 ns RTT 
Median: 226 ns RTT 

rigtorp::SPSCQueue:
Mean: 40904 ops/ms 
Median: 41492 ops/ms 
Mean: 197 ns RTT 
Median: 198 ns RTT