feat: GetBulk method

[DRK3]

• Implemented in CouchDB, in-memory and mock stores.
• Renamed PutAll to PutBulk to be consistent with this new method (which cannot be called GetAll since that's an existing method name)
• Updated store PutBulk method comment since the old comment was slightly inaccurate in the case of the concrete CouchDB implementation.
• Also added missing memstore PutBulk tests.
• Removed ErrBookmarksNotSupported from storage package since it's unused.

Signed-off-by: Derek Trider Derek.Trider@securekey.com

[codecov[bot]]

Codecov Report

Merging #125 (214ff27) into master (a61e670) will increase coverage by 0.59%. The diff coverage is 90.69%.

@@            Coverage Diff             @@
##           master     #125      +/-   ##
==========================================
+ Coverage   86.09%   86.68%   +0.59%     
==========================================
  Files          25       25              
  Lines        1410     1450      +40     
==========================================
+ Hits         1214     1257      +43     
+ Misses        121      117       -4     
- Partials       75       76       +1     

Impacted Files	Coverage Δ
pkg/storage/mysql/mysqlstore.go	78.57% <33.33%> (-0.44%)	:arrow_down:
pkg/storage/couchdb/couchdbstore.go	84.43% <93.75%> (+0.93%)	:arrow_up:
pkg/storage/memstore/memstore.go	100.00% <100.00%> (+10.52%)	:arrow_up:

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