Size-based throttling in Bookie side

[ArvinDevel]

feature request

FEATURE REQUEST

1. Please describe the feature you are requesting.
   Size based throttling in Bookie side

2. Indicate the importance of this issue to you (blocker, must-have, should-have, nice-to-have). Are you currently using any workarounds to address this issue?
   nice-to-have

3. Provide any additional detail on your proposed use case for this feature.
   Current bookie use maxPendingAddRequestsPerThread to limit server request processor‘s writing capacity, which can't deal different request size gracefully, and we can't setting it according to server resource accurately. In my test case, I have to change this setting to avoid the TooManyRequestsException.

[dlg99]

see #1409 / #1410