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