Merge #12493: [wallet] Reopen CDBEnv after encryption instead of shutting down

c1dde3a949 No longer shutdown after encrypting the wallet (Andrew Chow)
d7637c5a3f After encrypting the wallet, reload the database environment (Andrew Chow)
5d296ac810 Add function to close all Db's and reload the databae environment (Andrew Chow)
a769461d5e Move BerkeleyEnvironment deletion from internal method to callsite (Andrew Chow)

Pull request description:

  This is the replacement for #11678 which implements @ryanofsky's [suggestion](https://github.com/bitcoin/bitcoin/pull/11678#pullrequestreview-76464511).

  Shutting down the software was to prevent the BDB environment from writing unencrypted private keys to disk in the database log files, as was noted [here](https://bitcointalk.org/index.php?topic=51474.msg616068#msg616068). This PR replaces the shutdown behavior with a CDBEnv flush, close, and reopen which achieves the same effect: everything is cleanly flushed and closed, the log files are removed, and then the environment reopened to continue normal operation.

  To ensure that no unencrypted private keys are in the log files after encrypting the wallet, I wrote [this script](https://gist.github.com/achow101/7f7143e6c3d3fdc034d3470e72823e9d) to pull private keys from the original wallet file and searches for these keys in the log files (note that you will have to change your file paths to make it work on your own machine).

  As for concerns about private keys being written to slack space or being kept in memory, these behaviors no longer exist after the original wallet encryption PR and the shutting down solution from 2011.

  cc @ryanofsky

Tree-SHA512: 34b894283b0677a873d06dee46dff8424dec85a2973009ac9b84bcf3d22d05f227c494168c395219d9aee3178e420cf70d4b3eeacc9785aa86b6015d25758e75

src/wallet/rpcwallet.cpp

@@ -2121,7 +2121,6 @@ static UniValue encryptwallet(const JSONRPCRequest& request)
             "will require the passphrase to be set prior the making these calls.\n"
             "Use the walletpassphrase call for this, and then walletlock call.\n"
             "If the wallet is already encrypted, use the walletpassphrasechange call.\n"
-            "Note that this will shutdown the server.\n"
             "\nArguments:\n"
             "1. \"passphrase\"    (string) The pass phrase to encrypt the wallet with. It must be at least 1 character, but should be long.\n"
             "\nExamples:\n"
@@ -2159,11 +2158,7 @@ static UniValue encryptwallet(const JSONRPCRequest& request)
         throw JSONRPCError(RPC_WALLET_ENCRYPTION_FAILED, "Error: Failed to encrypt the wallet.");
     }
 
-    // BDB seems to have a bad habit of writing old data into
-    // slack space in .dat files; that is bad if the old data is
-    // unencrypted private keys. So:
-    StartShutdown();
-    return "wallet encrypted; Bitcoin server stopping, restart to run with encrypted wallet. The keypool has been flushed and a new HD seed was generated (if you are using HD). You need to make a new backup.";
+    return "wallet encrypted; The keypool has been flushed and a new HD seed was generated (if you are using HD). You need to make a new backup.";
 }
 
 static UniValue lockunspent(const JSONRPCRequest& request)