[build] Move wallet load functions to wallet/load unit

Moves the following wallet load functions to a new wallet/load unit in
the libbitcoin_wallet library. All other functions in wallet/init remain
in libbitcoin_server:

- `VerifyWallets`
- `LoadWallets`
- `StartWallets`
- `FlushWallets`
- `StopWallets`
- `UnloadWallets`

build_msvc/libbitcoin_server/libbitcoin_server.vcxproj.in

@@ -151,6 +151,9 @@
   </ItemDefinitionGroup>
   <ItemGroup>
 @SOURCE_FILES@
+    <ClCompile Include="..\..\src\wallet\init.cpp">
+      <ObjectFileName>$(IntDir)wallet_init.obj</ObjectFileName>
+    </ClCompile>
   </ItemGroup>
   <ItemGroup>
     <None Include="packages.config" />