Adjust ledger-tool bigtable upload starting-slot default value

Currently, if starting-slot is unspecified, a value of 0 will be chosen.
In the common case where someone is operating on a range much more
recent, this will result in a ton of wasted operations.

Instead, choose a smarter default value for starting-slot based on what
we detect is currently in the blockstore.

ledger-tool/src/bigtable.rs

@@ -38,7 +38,7 @@ use {
 
 async fn upload(
     blockstore: Blockstore,
-    mut starting_slot: Slot,
+    starting_slot: Option<Slot>,
     ending_slot: Option<Slot>,
     force_reupload: bool,
     config: solana_storage_bigtable::LedgerStorageConfig,
@@ -53,6 +53,16 @@ async fn upload(
     };
     let blockstore = Arc::new(blockstore);
 
+    let starting_slot = if let Some(starting_slot) = starting_slot {
+        Ok(starting_slot)
+    } else {
+        // It is possible that the slot returned here could get pruned before
+        // the value is later used by upload_confirmed_blocks(). This is ok as
+        // that method doesn't need the exact slot to exist; rather, the slot
+        // returned here is used as a lower bound to search for roots.
+        blockstore.get_first_available_block()
+    };
+    let mut starting_slot = starting_slot?;
     let ending_slot = ending_slot.unwrap_or_else(|| blockstore.last_root());
 
     while starting_slot <= ending_slot {
@@ -640,7 +650,7 @@ pub fn bigtable_process_command(
 
     let future = match (subcommand, sub_matches) {
         ("upload", Some(arg_matches)) => {
-            let starting_slot = value_t!(arg_matches, "starting_slot", Slot).unwrap_or(0);
+            let starting_slot = value_t!(arg_matches, "starting_slot", Slot).ok();
             let ending_slot = value_t!(arg_matches, "ending_slot", Slot).ok();
             let force_reupload = arg_matches.is_present("force_reupload");
             let blockstore = crate::open_blockstore(