* Extensible split data sync capability through transactions.
- Split common transport has been split up between the transport layer
and data layer.
- Split "transactions" model used, with convergence between I2C and
serial data definitions.
- Slave matrix "generation count" is used to determine if the full slave
matrix needs to be retrieved.
- Encoders get the same "generation count" treatment.
- All other blocks of data are synchronised when a change is detected.
- All transmissions have a globally-configurable deadline before a
transmission is forced (`FORCED_SYNC_THROTTLE_MS`, default 100ms).
- Added atomicity for all core-synced data, preventing partial updates
- Added retries to AVR i2c_master's i2c_start, to minimise the number of
failed transactions when interrupts are disabled on the slave due to
atomicity checks.
- Some keyboards have had slight modifications made in order to ensure
that they still build due to firmware size restrictions.
* Fixup LED_MATRIX compile.
* Parameterise ERROR_DISCONNECT_COUNT.
