WIP
WIP
WIP
Finished sphinx support
Added Connection::useMaster() and refactored code.
Implemented master balancing.
do not switch connection when in a transaction.
Fixed test brak.
Finished documentation about read-write splitting.
test WIP
Added test.
all finished. [skip ci]
Merge pull request #4274 from yiisoft/feature-multiconnection

Support for separating Read/Write Database
Updated history [skip ci]
Update intro-upgrade-from-v1.md
Merge pull request #4289 from mrarthur/patch-2

Error in code example
Typos and errors in docs
Merge pull request #4291 from mrarthur/patch-3

Typos and errors in docs
Edited; already very clean!
Edited; very clean!