Add in flight counting (enables backpressure)

[alanshaw]

In the upload service, track claimed and used capacity per the document here: https://www.notion.so/storacha/Service-Provider-Selection-2f05305b55248080af09c97b6556d5d8

Implement limits for claimed size that automatically excludes nodes from selection when exceeded.

Service should automatically remove unused claimed size i.e. an allocation that was never accepted.