(#2907) - Updating 'Common Errors' page

...with 'No valid adapter found' error, and 'Unknown
(Chrome/Opera)' error explanations.
(#3160) - Remove redundant jshint dir
(#3136) - fix changes() when earlier winning seq

In the previous version of the changes() algorithm,
we were assuming that the winning seq was later than
all non-winning seqs, which isn't necessarily the case
when you've got conflicts. This fixes that.
(#3169) - edit guide to changes

Now that I understand `last_seq` better, I understand
that it is not a footgun.
(#3089) - truly auto auto-compaction
(#3015) - Issues GET requests on pull replication in parallel for each batch.

In node.js using default settings (5 connections in the pool + 100 docs per batch) over a HTTP connection to a local PouchDB server perf increased by 8x over previous perf.

When replicating down the 0.6 gig NPM CouchDB database over HTTPS using CouchDB locally it took CouchDB about 0.69 seconds/100 records. Without this change it took PouchDB around 30 seconds/100 records. With this change on default settings we reduced time to 4 seconds/100 records. By changing the number of connections to 15 and the batch size to 1000 we got perf down to 0.9 seconds/100 records. So perf got down to only 30% worse than CouchDB. I strongly suspect that when the node.js connection harvesting bug is fixed (so we don't lose our existing connections between batches) and with a little more tweaking (it's clear that we are a bit more 'chatty' than CouchDB's replication client) we should be able to match CouchDB's perf.
(#3094) - Add easy to run couchdb server with Docker and Fig

update CONTRIBUTING and TESTING accordingly
remove test part from CONTRIBUTING, link to TESTING, DRY