check if the slashing protection database is locked before creating keys (#1949)

## Issue Addressed

Closes #1790

## Proposed Changes

Make a new method that creates an empty transaction with `TransactionBehavior::Exclusive` to check whether the slashing protection is locked. Call this method before attempting to create or import new validator keystores.  

## Additional Info

N/A


Co-authored-by: realbigsean <seananderson33@gmail.com>

account_manager/src/validator/create.rs

@@ -192,6 +192,14 @@ pub fn cli_run<T: EthSpec>(
             )
         })?;
 
+    // Create an empty transaction and drops it. Used to test if the database is locked.
+    slashing_protection.test_transaction().map_err(|e| {
+        format!(
+            "Cannot create keys while the validator client is running: {:?}",
+            e
+        )
+    })?;
+
     for i in 0..n {
         let voting_password = random_password();
         let withdrawal_password = random_password();

account_manager/src/validator/import.rs

@@ -86,6 +86,14 @@ pub fn cli_run(matches: &ArgMatches, validator_dir: PathBuf) -> Result<(), Strin
             )
         })?;
 
+    // Create an empty transaction and drop it. Used to test if the database is locked.
+    slashing_protection.test_transaction().map_err(|e| {
+        format!(
+            "Cannot import keys while the validator client is running: {:?}",
+            e
+        )
+    })?;
+
     eprintln!("validator-dir path: {:?}", validator_dir);
     // Collect the paths for the keystores that should be imported.
     let keystore_paths = match (keystore, keystores_dir) {