(#1853) - make perf tests more accurate for skip/limit

This was accidentally reverted by
fd2a695f4d5ca98dcc8d4f66a2eeabcd437939a6.
(#1897) - idb perf improvements for get/allDocs

Similar to what was done for WebSQL, this caches
the docCount as well as minimizing the calls to
isDeleted and merge.winningRev. The winningRev
is also stored in the by-seq store, similar to how
the winningSeq is stored in websql.