It's important to say that this bug apparently (till now) only happens
when an *incorrect* instalation of a site happens, reusing the dataroot
from another existing site.

When that happens, the localcache/bootstrap.php file from the old
site is reused, setting siteidentifier and SYSCONTEXTID when it's not
time for them to be defined yet.

Their existenece leads to reusing some other structures from the old
dataroot (that, again, should have been changed or wiped!), ultimately
leading to all sort of errors about non-existent tables (course,
context...).

With this change we ensure that:

1) Whenever any change to the database configuration (prefix, user,
   type..) happens, it's detected and immediately the information
   in the localcache/bootstrap.php is discarded and the file removed.
   This should fix problems like MDL-73098 itself.

2) We only set SYSCONTEXTID if the file is not stale. Main reason
   for doing that check within the localcache/bootstrap.php file
   itself is that we cannot "undefine" it @ setup.php. This should
   prevent errros like MDL-72888 to happen.

3) Finally, little detail, we only define SYSCONTEXTID if it has
   not been defined earlier. In the past, it was recommended to
   define it in config.php (exactly to save one DB read) and, sites
   having them will face "already defined" warnings.