Use libmemcached benchmark memaslap

[Sergey Bronnikov]

libmemcached has a benchmark memaslap, and after commit 7a7cca8 it is built with target libmemcached_copy. Probably it makes sense to add it for testing memcached.

Documentation:

• http://docs.libmemcached.org/bin/memaslap.html
• http://docs.libmemcached.org/bin/memslap.html

 memcslap --servers=127.0.0.1:8080 --binary --verbose --test=get --flush --non-blocking --concurrency=1000 --initial-load=100 --debug 

Depends on #90