replace tempdir by tempfile (#2143)

## Issue Addressed

Fixes #2141 
Remove [tempdir](https://docs.rs/tempdir/0.3.7/tempdir/) in favor of [tempfile](https://docs.rs/tempfile/3.1.0/tempfile/).

## Proposed Changes

`tempfile` has a slightly different api that makes creating temp folders with a name prefix a chore (`tempdir::TempDir::new("toto")` => `tempfile::Builder::new().prefix("toto").tempdir()`).

So I removed temp folder name prefix where I deemed it not useful.

Otherwise, the functionality is the same.

validator_client/src/validator_store.rs

@@ -8,7 +8,7 @@ use slog::{crit, error, warn, Logger};
 use slot_clock::SlotClock;
 use std::path::Path;
 use std::sync::Arc;
-use tempdir::TempDir;
+use tempfile::TempDir;
 use types::{
     Attestation, BeaconBlock, ChainSpec, Domain, Epoch, EthSpec, Fork, Hash256, Keypair, PublicKey,
     SelectionProof, Signature, SignedAggregateAndProof, SignedBeaconBlock, SignedRoot, Slot,