The new benchmark test for blobs fs backend was rebased from a merge
request in which it was written using the new interface for blobs
backend. After rebase, the test was not reverted to old interface, and
was just failing bad. This commit fixes it.

-- Closes: #9004

It was generating spaces, causing split to fail sometimes.

Adds a CooperativeTask to stream blobs to client as the transport is
able to handle it, pausing and resuming as necessary.

-- Related: #8809

First version, still missing consumer/producer model and some tweaks,
but working.

-- Related: #8809

-- Related: #8809

-- Related: #8809

Closes: #8691

Intercept the creation of the protocol factory in the HTTP connection
pool to use twisted.protocols.policies.ThrottlingFactory and control the
incoming and outgoing bandwidth.

The factory only controls one connection, so when throttling we limit
the number of connections of the pool to one per host. This way,
throttling happens in a per-host basis.

Closes: #8931

Accessing _all_dbs slows down the test and open possibilites for
heisenbugs where some old leftover database can change code behavior.

-- Resolves: #9001