Fix Olog HttpClient timeout and URL encoding

[georgweiss]

URLEncoder.encode() converts space char to +, which does not work if the string is a path variable.
Suggest to change the API for fetching attachments to use the (MongoDB) id instead of original file name, but this will require a change in the service.

All in all the native HttpClient is a bit of a challenge to work with... Or: the Jersey client makes a better job keeping the developer happy.

Also optimizes instantiation of LogEntryTable app to avoid multiple calls to create a OlogHttpClient.

See #3236

[shroffk]

I expected the app.getClient() to always return the same instance if the client had been instantiated along the lines of a singleton pattern
This is more explicit though