2.5.x branch should not contain the PHP 5.5-only ::class syntax

Close #2730

Merge branch 'fix/#2384-fluent-methods-in-query-builder-are-now-documented-as-returning-this' into 2.5

Backport #2384 to 2.5.x

Close #2686
Close #889

primary key on MySQL platform.

Merge branch 'fix/#2644-#2647-fix-fetchAll-with-pdo-fetch-column-on-db2-with-portability-wrapper' into 2.5

Backport #2644 to 2.5.x
Backport #2647 to 2.5.x

Backport #2405 to 2.5.x
Backport #2654 to 2.5.x

I am also of the opinion that the resource type checks are unhappy. We depend on driver internals.

removed unnecessary  setup/teardown-methods

* removed "charset" patch
* implemeted resource_type compare with in_array instead of regex
* implemented functional tests for connection and statement.

Revert "build a correct connection string for mirrored databases, too. In this case "host=ip1:port,ip2:port" servername and dbname are optional but can also be used without host parameter"

This reverts commit 0698cba6.

build a correct connection string for mirrored databases, too. In this case "host=ip1:port,ip2:port"
servername and dbname are optional but can also be used without host parameter

if $persistent=false, resource_type == 'SQLAnywhere connection'

Backport #2650 to 2.5.x

Backport #2651 to 2.5.x

Backport #2637 to 2.5.x
Backport #2645 to 2.5.x