(#3211) - remove unused metadata.seq index (idb)

So this is a bit of a hard choice, because if we ever want to bring this index back, it will have to be in a migration.

Besides the perf numbers below (which are good), my reasoning is thus:

I already tested using the `metadata.seq` for `changes()`, under the hypothesis that it would be better to only iterate winning seqs. In fact the perf [was actually worse for `temp-views`](https://github.com/pouchdb/pouchdb/issues/3182#issuecomment-66940346), presumably because the cost of using the secondary index outweighed the benefit of increasing the read perf, which makes sense, since in those tests, all the docs are generation-1 and so there are no `seq`s to be skipped.

So there is an argument to be made that for databases with lots of revisions to a small number of documents, it would be faster to iterate over `metadata.seq` than `seq`. However, since most databases only need to use `changes()` for replication, which uses checkpoints, meaning it only has to iterate once, I think this is fine.

Plus, writes are slower than reads in LevelDB under the hood, and writes are what we want to really optimize for. Our read performance seems good, in general.

Anyway, perf numbers for this commit are below. Note that I bumped up the number of iterations for the `insert` tests in order to get more granularity:

| browser | test | iterations | before (ms) | after (ms) | improvement |
| --- | --- | --- | --- | --- | --- |
Chrome 39 | basic-inserts | 10000 | 34749 | 28502 | 17.9774957553% |
Firefox 36 | basic-inserts | 10000 | 85217 | 78201 | 8.23309902954% | 
Chrome 39 | bulk-inserts | 1000 | 60545 | 55115 | 8.96853579982% | 
Firefox 36 | bulk-inserts | 1000 | 152519 | 133135 | 12.7092362263% |
Chrome 39 | temp-views | 1 | 11046 | 10984 | 0.561289154445% |
Firefox 36 | temp-views | 1 | 19319 | 19041 | 1.43899787774% |