Move Item member initialization to initializer list

More refactoring to prepare for merging consturctors, also is more
efficient as we don't default construct the std::string for key before
assigning it.

Change-Id: I3c6e45ea9f3303e5d29f7231ae282f1d59efb31c
Reviewed-on: http://review.couchbase.org/42624
Reviewed-by: Michael Wiederhold <mike@couchbase.com>
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>
Reviewed-by: abhinav dangeti <abhinav@couchbase.com>
Tested-by: abhinav dangeti <abhinav@couchbase.com>