Memcached: preallocate slice used for grouping cache keys by server

[56quarters]

What this PR does:

We know the number of keys and so should preallocate the slice used. Runtime of both the new and old version varies by a lot however, the new version tends to be faster (in addition to consistently allocating less memory).

$ benchstat -delta-test=none old.txt new.txt 
name                                 old time/op    new time/op    delta
MemcachedClient_sortKeysByServer-16    2.53ms ± 0%    2.11ms ± 0%  -16.60%

name                                 old alloc/op   new alloc/op   delta
MemcachedClient_sortKeysByServer-16    1.31MB ± 0%    0.89MB ± 0%  -32.23%

name                                 old allocs/op  new allocs/op  delta
MemcachedClient_sortKeysByServer-16     20.1k ± 0%     20.1k ± 0%   -0.02%

Which issue(s) this PR fixes:

N/A

Checklist

• Tests updated
• CHANGELOG.md updated - the order of entries should be [CHANGE], [FEATURE], [ENHANCEMENT], [BUGFIX]

[pracucci]

Nice work!