Block user from changing "eviction policy" at runtime

With current 3.0 installations, the user can switch between full and value eviction after a bucket is created, data loaded etc.

Per PM spec, this behaviour should be blocked.

Confirmed the above using cbstats .. | grep eviction

[root@cherry-s10105 bin]# ./cbstats localhost:11210 -b b-1 all | grep eviction
ep_item_eviction_policy: value_only

Change to "fulleviction" back from UI, and check the cbstats
[root@cherry-s10105 bin]# ./cbstats localhost:11210 -b b-1 all | grep eviction
ep_item_eviction_policy: full_eviction