Adjust zebrad Buffer sizes, and document Buffer constraints

[teor2345]

Is your feature request related to a problem? Please describe.

zebrad uses a lot of Buffered services, but we haven't really checked that their bounds are high enough.

The underlying issues are:

• If lots of tasks with data dependencies use a Buffer or Batch with a small bound, Zebra can hang.
• Buffers or Batches with small bounds can also limit concurrency, particularly if the underlying service does a lot of work in its call.

Describe the solution you'd like

• Review Buffer bounds across zebrad
• Consider memory denial of service risks, particularly for the inbound buffer, inbound downloads buffer, and sync downloads buffer

Document the Buffer constraints in:

• commands::start
• components::sync::ChainSync
• components::sync::Inbound

See #1593 for details - ideally we should point to an RFC here.

Describe alternatives you've considered

Do nothing: the code might hang if future changes break the poll_ready/call invariants.

Additional context

This issue was discovered during the review in #1593. It is a routine cleanup.

[teor2345]

This task has been revised based on the fixes to the underlying Buffer issue, see #1593 for details.

[mpguerra]

we will create other issues as needed