Moves key management functions into a new class LegacyScriptPubKeyMan. First two commits are move-only commits which move stuff out of wallet.{h/cpp} and into newly created scriptpubkeyman.{h/cpp}. Third commit changes several things in CWallet to use LegacyScriptPubKeyMan.
First step in the wallet boxes refactor. Note that LegacyScriptPubKeyMan and ScriptPubKeyMan cannot be used standalone yet and are still very much tied into CWallet with both accessing functions within each other. This PR is to help reduce review burden.