[TieredStorage] Add capacity() API and limit file size to 16GB (#401)

#### Problem
The TieredStorage has not yet implemented the AccountsFile::capacity()
API.

#### Summary of Changes
Implement capacity() API for TieredStorage and limit file size to 16GB,
same as the append-vec file.

accounts-db/src/tiered_storage.rs

@@ -38,6 +38,8 @@ use {
 
 pub type TieredStorageResult<T> = Result<T, TieredStorageError>;
 
+const MAX_TIERED_STORAGE_FILE_SIZE: u64 = 16 * 1024 * 1024 * 1024; // 16 GiB;
+
 /// The struct that defines the formats of all building blocks of a
 /// TieredStorage.
 #[derive(Clone, Debug, PartialEq)]
@@ -163,6 +165,11 @@ impl TieredStorage {
     pub fn is_empty(&self) -> bool {
         self.len() == 0
     }
+
+    pub fn capacity(&self) -> u64 {
+        self.reader()
+            .map_or(MAX_TIERED_STORAGE_FILE_SIZE, |reader| reader.capacity())
+    }
 }
 
 #[cfg(test)]

accounts-db/src/tiered_storage/hot.rs

@@ -369,6 +369,10 @@ impl HotStorageReader {
         self.len() == 0
     }
 
+    pub fn capacity(&self) -> u64 {
+        self.len() as u64
+    }
+
     /// Returns the footer of the underlying tiered-storage accounts file.
     pub fn footer(&self) -> &TieredStorageFooter {
         &self.footer

accounts-db/src/tiered_storage/readable.rs

@@ -44,6 +44,12 @@ impl TieredStorageReader {
         }
     }
 
+    pub fn capacity(&self) -> u64 {
+        match self {
+            Self::Hot(hot) => hot.capacity(),
+        }
+    }
+
     /// Returns the footer of the associated HotAccountsFile.
     pub fn footer(&self) -> &TieredStorageFooter {
         match self {