Move sending of data within `transceive` method.

During tests it occured that the timer was created AFTER the `done`
callback has already been called. That results in unnecessary retries.
Performing the transmission call after the timer creation ensures that
once the `done` callback is called, the timer will be cleared as
intended.
Add connection state check to transceiveConfiguration.

Previously the transmission of value requests continued even when the
underlying connection has been interrupted or disconnected. This patch
adds checks and suspends transceiving while the connection is trying
to reestablish.
Complete ConnectionCustomizer tests.