This PR modularizes `manager.{hh, cc}` by dividing the files into separate smaller units. The changes improve overall readability of code and help reason about it. Each file has a specific purpose now.
This is the first step in refactoring the Hinted Handoff module.
Refs scylladb/scylla#15358Closesscylladb/scylladb#15378
* github.com:scylladb/scylladb:
db/hints: Remove unused aliases from manager.hh
db/hints: Rename end_point_hints_manager
db/hints: Rename sender to hint_sender
db/hints: Move the rebalancing logic to hint_storage
db/hints: Move the implementation of sender
db/hints: Move the declaration of sender to hint_sender.hh
db/hints: Move sender::replay_allowed() to the source file
db/hints: Put end_point_hints_manager in internal namespace
db/hints: Move the implementation of end_point_hints_manager
db/hints: Move the declaration of end_point_hints_manager
db/hints: Move definitions of functions using shard hint manager
db/hints: Introduce hint_storage.hh
db/hints: Extract the logger from manager.cc
db/hints: Extract common types from manager.hh
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