Improve featureUtility performance with remote repository

[jjiwooLim]

FeatureUtility can take a long time to resolve features when using a remote repository with a slow network.

Areas that could be improved:

1. Check and retrieve more than one checksums from maven repository
2. Check more than one artifact urls for missing features

[jdmcclur]

3. Look into caching assets instead of jsonobjects and caching per file so a file doesn't need to be parsed/deserialized more than once. This won't help with a slow network, but will speed things up.
   

   open-liberty/dev/com.ibm.ws.repository/src/com/ibm/ws/repository/transport/client/SingleFileClient.java

   Line 66 in ab5f0ec

   private Map<String, JsonObject> assets;

[jdmcclur]

4. Download more than 1 file at once -

   open-liberty/dev/com.ibm.ws.install/src/com/ibm/ws/install/internal/ArtifactDownloader.java

   Line 100 in 5e6910d

   for (String coords : mavenCoords) {

[jjiwooLim]

5. Changing SingleFileRepositoryConnection to cache the client between calls to createClient would significantly reduce the json parsing we do. It looks like the SingleFileClient class already detects changes to the underlying file and repress as necessary so I think this would be safe:

open-liberty/dev/com.ibm.ws.repository/src/com/ibm/ws/repository/connections/SingleFileRepositoryConnection.java

Line 39 in 43eeef1

public RepositoryReadableClient createClient() {

[jjiwooLim]

6. Looking at the feature.json file it seems that the json contains the sha256 of the downloaded esa file so there is no need to download the sha hash from maven central because we can already validate the download using the json content.