docs: add README to AccountsDB (#357)

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## Greptile Summary

Added a new README.md file documenting the AccountsDB component, which
provides essential documentation for the database's features and API
methods.



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closes gh-330

---------

Co-authored-by: greptile-apps[bot] <165735046+greptile-apps[bot]@users.noreply.github.com>

Author: bmuddha

magicblock-accounts-db/README.md

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+# Accounts Database Manager
+
+The AccountsDB is a crucial component of the Magicblock Validator. It handles
+the storage, retrieval, and management of account data. It's highly optimized
+to provide efficient account search and retrieval, using memory mapped storage,
+while still being persisted to disk.
+
+## Features
+
+- **Account Storage**: Efficient storage of account records, with support for
+  both owned and borrowed account data. By default accounts are read directly
+  from memory mapping without any deserialization and/or allocation, as
+  consequence it allows for direct memory modification in situ in the database,
+  avoiding read modify write overhead. But this imposes a strict requirement, that
+  no two threads have access to the same account at the same time for
+  modification, which is naturally achieved with account locking when executing
+  transactions. But any other way to hold onto borrowed state of account is
+  prohibited, as it will inevitably cause an undefined behavior.
+- **Index Management**: Fast index-based lookups for account data.
+  LMDB is used for the purposes of indexing, where several different mappings
+  are maintained to support database integrity.
+- **Snapshot Management**: Automated snapshot creation to enable rollbacks to
+  previous states of the database if necessary.
+- **Concurrency Control**: Uses a "Stop the World" lock to ensure data
+  consistency during critical operations like snapshotting. This prevents any
+  writes to the database while it's being manipulated at the OS level. 
+- **Program Account Scanning**: Retrieve and filter accounts associated with a
+  specific program, the scanning happens without deserializing anything from the
+  database.
+
+## Main API methods
+
+- **Initialization**: Use `AccountsDb::new` to create or open an accounts
+  database instance.
+- **Account Operations**: Use `get_account`, `insert_account`,
+  `contains_account` to read, write, or check the existence of accounts.
+- **Snapshot Operations**: Schedule and manage snapshots with `set_slot`, which
+  might trigger snapshot operation, which in turn involves locking everything
+  down, thus snapshot frequency should be set to sane value. And retrieve
+  snapshot slots using `get_latest_snapshot_slot` and
+  `get_oldest_snapshot_slot`.
+- **Database Integrity**: Use `ensure_at_most` to ensure database state up to a
+  specific slot, with rollback to previously taken snapshot if needed.