replace credit_flow:update/3 with a macro
un order to eliminate closure creation and thus improve performance.

Also...
- get rid of a 'get/2' call by processing result of 'erase' instead
- inline remaining get/2 calls for performance
get rid of rabbit_channel MASKED_CALL macro
which simplifies the code and eliminates a performance hotspot
Explain
Merge bug25364.
Typo
Merge bug25366