Fix doc on 'make cold'

Closes #1873
Trim package messages

makes it easier to layout in opam files
Consistent cleanup of temp files for parallel and seq processes
Allow pin --kind http, since it's now supported
Warnings and spelling
Fixed compat printer for 1.1

there still weren't enough parens
Merge pull request #1890 from OCamlPro/various-fixes

Fixes compat mode, some doc and spelling, allows pin --kind http
Implement mixed-mode VC pin

Closes #1868. Thanks a lot for the idea !
Document pin "mixed mode"
Fix mixed mode: sync the versionned files at source, not dest
Merge pull request #1888 from OCamlPro/mixed-vc-pin

Implement mixed-mode VC pin
Remove debug
Merge pull request #1892 from OCamlPro/oops-debug

Remove debug
Simplify and speedup repo update

All the md5s were recomputed at each 'opam update', which is unnecessary since we already
have a URL file. Rebuilding the package cache is still expensive (until the OS cache is full) though

Bench with
```
sudo sysctl vm.drop_caches=3
time opam update
```
Cleanup: no need for preload anymore

we don't have concurrency issues anymore, so it's fine to get the repo
state cache lazily (as long as we don't yield in the middle)
Merge pull request #1893 from OCamlPro/faster-update

Simplify and speedup repo update, don't recompute md5s each time
Documentation refresh

Including:
- link to the html dev-manual
- promote opam-publish rather than the more complex instructions
- document workaround for OPAM 1.1 in the FAQ
- add the 'tricks' page
Merge pull request #1894 from OCamlPro/doc-refresh

Documentation refresh
Doc pages: some formatting issues
Propose to save back a modified opam file in mixed VC pin mode