release-20.1: schemachange: speed up slow schema changes

[spaskob]

Backport 1/1 commits from #48608.

/cc @cockroachdb/release

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Touches #45150.
Fixes #47607.
Touches #47790.

Release note (performance improvement):
Before this a simple schema change could take 30s+.
The reason was that if the schema change is not first
in line in the table mutation queue it would return a
re-triable error and the jobs framework will re-adopt and
run it later. The problem is that the job adoption loop
is 30s.

To repro run this for some time:

cockroach sql --insecure --watch 1s -e 'drop table if exists users cascade; create table users (id uuid not null, name varchar(255) not null, email varchar(255) not null, password varchar(255) not null, remember_token varchar(100) null, created_at timestamp(0) without time zone null, updated_at timestamp(0) without time zone null, deleted_at timestamp(0) without time zone null); alter table users add primary key (id); alter table users add constraint users_email_unique unique (email);'

Instead of returning on re-triable errors we retry with exponential
backoff in the schema change code. This pattern of dealing with
re-triable errors in client job code is encouraged vs relying on the
registry because the latter leads to slowness and additionally to more
complicated test fixtures that rely on hacking with the internals of the
job registry,

[cockroach-teamcity]

This change is 

[ajwerner]

too

I'm going to push the button

Reviewable status: complete! 1 of 0 LGTMs obtained (waiting on @ajwerner)