box: allow upgrading from version 1.6

Direct upgrade support from pre-1.7.5 versions was removed in commit
7d3b80e7
(Forbid upgrade from Tarantool < 1.7.5 and refactor upgrade.lua)
The reason for that was the mandatory space format checks introduced
back then. With these space format checks, old schema couldn't be
recovered on new Tarantool versions, because newer versions had
different system space formats. So old schema couldn't be upgraded
because it couldn't even be recovered.

Actually this was rather inconvenient. One had to perform an extra
upgrade step when upgrading from, say, 1.6 to 2.x: instead of
performing a direct upgrade one had to do 1.6 -> 1.10 -> 2.x upgrade
which takes twice the time.

Make it possible to boot from snapshots coming from Tarantool version
1.6.8 and above.

In order to do so, introduce before_replace triggers on system spaces,
which work during snapshot/xlog recovery. The triggers will set tuple
formats to the ones supported by current Tarantool (2.x). This way the
recovered data will have the correct format for a usual schema upgrade.

Also add upgrade_to_1_7_5() handler, which finishes transformation of
old schema to 1.7.5. The handler is fired together with other
box.schema.upgrade() handlers, so there's no user-visible behaviour
change.

Side note: it would be great to use the same technique to allow booting
from pre-1.6.8 snapshots. Unfortunately, this is not possible.

Current triggers don't break the order of schema upgrades, so 1.7.1
upgrades come before 1.7.2 and 1.7.5. This is because all the upgrades
in these versions are replacing existing tuples and not inserting new
ones, so the upgrades may be handled by the before_replace triggers.

Upgrade to 1.6.8 requires inserting new tuples: creating sysviews, like
_vspace, _vuser and so on. This can't be done from the before_replace
triggers, so we would have to run triggers for 1.7.x first which would
allow Tarantool to recover the snapshot, and then run an upgrade handler for
1.6.8. This looks really messy.

Closes #5894