Bug 10611: Use mysql_auto_reconnect instead of ping

DBD::Mysql provides a mysql_auto_reconnect flag. Using it avoids
the time required to do a $dbh->ping().

Benchmarks:

use Modern::Perl;
use C4::Context;
for ( 1 .. 1000 ) {
    $dbh = C4::Context->dbh;
}

* without this patch on a local DB:
perl t.pl  0,49s user 0,02s system 98% cpu 0,525 total
* without this patch on a remote DB:
perl t.pl  0,52s user 0,05s system 1% cpu 37,358 total
* with this patch on a local DB:
perl t.pl  0,46s user 0,04s system 99% cpu 0,509 total
* with this patch on a remote DB:
perl t.pl  0,49s user 0,02s system 56% cpu 0,892 total

Testing the auto reconnect:
use Modern::Perl;
use C4::Context;
my $ping = $dbh->ping;
say $ping;
$dbh->disconnect;
$ping = $dbh->ping;
say $ping;

Signed-off-by: Bernardo Gonzalez Kriegel <bgkriegel@gmail.com>
Comment: Real improvement. No koha-qa errors

prove t/db_dependent/Circulation_issuingrules.t produces no error
prove t/db_dependent/Context.t produces no error

Test
1) dumped Koha DB, load it on a non-local server
2) run sample script whit and without patch, local and remote

use Modern::Perl;
use C4::Context;
for ( 1 .. 100000 ) {
    my $dbh = C4::Context->dbh;
}

Main difference I note is with remote server
a) without patch
real    0m16.357s
user    0m2.592s
sys     0m2.132s

b) with patch
real    0m0.259s
user    0m0.240s
sys     0m0.012s

I think this could be good for DBs placed on
remote servers

Bug 10611: add a "new" parameter to C4::Context->dbh

When dbh->disconnect is called and the mysql_auto_reconnect flag is set,
the dbh is not recreated: the old one is used.

Adding a new flag, we can now force the C4::Context->dbh method to
return a new dbh.

Signed-off-by: Bernardo Gonzalez Kriegel <bgkriegel@gmail.com>

Bug 10611: Followup: remove useless calls to dbh->disconnect

These 3 calls to disconnect are done at the end of the script, they are
useless.

Signed-off-by: Bernardo Gonzalez Kriegel <bgkriegel@gmail.com>
Signed-off-by: Paul Poulain <paul.poulain@biblibre.com>
Signed-off-by: Galen Charlton <gmc@esilibrary.com>
Bug 10611: (follow-up) cache DBMS driver code in context object

Rather than use a package-level variable, use an object-level
one.

Signed-off-by: Galen Charlton <gmc@esilibrary.com>