make installers: fixes for `parallel` mode and timeouts

The `parallel` form in a site configuration was working only
with leaf machines; now, it should be fixed for `sequential`
nested under `parallel`.

Also, fix timeout handling and reporting of failures.
(cherry picked from commit 266e4ab1191cb9d3304b73e20a0c36b15fdf148d)
apply signature in productbuild, not pkgbuild

Merge to release branch.
(cherry picked from commit 6f94215f92ccbc67e321b86544abbde109a23a4f)
history for 2htdp teachpacks
(cherry picked from commit 559af9b35c9ab39d09e2f1f301233a104a4b07ba)
add missing HISTORY entries based on my changes leading up to 6.0

Please merge to 6.0 (some of the history files already have
post-6.0 notes in them, but hopefully this patch still applies
cleanly)
(cherry picked from commit 4408cdcfe4566fe3f0b7970f0dec7435076a5f70)
fill in some missing history notes

Merge to v6.0
(cherry picked from commit dc526f95f30d72c2a854f9e1c7520f04cbdaf0d0)
Update HISTORY

Please merge to v6.0
(cherry picked from commit 8c6cf7395d4ee25debb66ba2731bf07d231d865a)
Add a HISTORY file for unstable

Note: I tried to reconstruct some history back
to 5.3.4, but it's not complete.

Please merge to v6.0
(cherry picked from commit c75e84a3720b69e47db99a255c98df1052562211)
Correct pkgs urls, please include in v6
(cherry picked from commit 2e5a89e68201fcf316321578f6394d20d7750c93)

Conflicts:
	pkgs/racket-pkgs/racket-doc/pkg/scribblings/getting-started.scrbl
update HISTORY based on some of Jay's changes

please include in 6.0
(cherry picked from commit 438942c0590841b030795d5df8db4c8409594cb4)
fix sqlite rollback-transaction in case of auto-rollback error

Merge to release branch.

An invalid (needs-rollback?) transaction may correspond to a closed
underlying transaction. In that case, don't issue ROLLBACK command.
(cherry picked from commit 06aa4770f13a739b31f5ae5c0e1434da05282598)
fix problem with doc database in read-only mode

Now that the documentation-build phase of `raco setup` uses read-only
mode, contention between writing and reading sometimes (though
infrequently) triggers an SQLITE_IOERR_LOCK error. Change the SQLite
binding to expose that specific error, as well as SQLITE_IOERR_BLOCKED,
because a retry of the affected operation is appropriate.
(cherry picked from commit 6858b8df90a33bdaa01cf9696db0e4be73374464)