issue-1146: preliminary changes: extract some localdb methods to the interface, add stub for index cache

[debnatkh]

Currently RO transactions access TIndexTabletDatabase via TIndexTabletState methods like

TIndexTabletState::ReadNode(
    TIndexTabletDatabase& db,
    ...

which in turn calls do stuff like db.ReadNode(nodeId, commitId, node).

We want to cache some index data from the Index tables.

For this in-memory cache to implement the same set of methods, TIndexTabletDatabase is replaced with an interface IIndexTabletDatabase , which is implemented by TInMemoryIndexState (which, for now, is a stub). Its superset, IIndexTabletDatabase is also implemented by TIndexTabletDatabase.

To ensure in-memory cache validity, every RW transaction that modifies inode index records all modifying operations, and on completion of those transactions, these operations will be applied to the in-memory state. This is done using TIndexTabletDatabaseProxy : public TIndexTabletDatabase (to be implemented), which forwards all requests to the underlying TIndexTabletDatabase.

classDiagram
TNiceDb <|-- TIndexTabletDatabase
TIndexTabletDatabase : actual implementation that accesses LocalDB
    IIndexTabletDatabase <|-- TInMemoryIndexState
class TInMemoryIndexState {
- ApplyDelta(changes to inmemory state)
}

    IIndexTabletDatabase <|-- TIndexTabletDatabase
    class IIndexTabletDatabase {
      + bool ReadNode()
      + bool ReadNodeVer()
      + bool ReadNodeAttr_s()
      + bool ReadNodeAttrVer_s()
      + bool ReadNodeRef_s()
      + bool PrechargeNodeRefs()
      + bool ReadNodeRefVer_s()
    }

TIndexTabletDatabase<|-- TIndexTabletDatabaseProxy : forwards all DB accesses
TIndexTabletDatabaseProxy: records all modifications of the tables
TInMemoryIndexState<|.. TIndexTabletDatabaseProxy : can apply recorded changes\nto in-memory state

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References #1146

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🟢 linux-x86_64-relwithdebinfo: all tests PASSED for commit 3243440.

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🔴 linux-x86_64-relwithdebinfo: some tests FAILED for commit 30aef9f.

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🟢 linux-x86_64-relwithdebinfo: all tests PASSED for commit 9225ee4.

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🟢 linux-x86_64-relwithdebinfo: all tests PASSED for commit 584851d.

TESTS	PASSED	ERRORS	FAILED	SKIPPED	MUTED?
1634	1634	0	0	0	0

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🟢 linux-x86_64-relwithdebinfo: all tests PASSED for commit b401a04.

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1634	1634	0	0	0	0

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🟢 linux-x86_64-relwithdebinfo: all tests PASSED for commit e7c00bc.

TESTS	PASSED	ERRORS	FAILED	SKIPPED	MUTED?
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