Add configuration option for setting solr endpoint by single connection string

[misino]

Single configuration option for solr endpoint will make integration with other projects easier.

[godong9]

I'm not sure about this is really need.

[misino]

There are 2 parts which are solved in this PR.

1. Current solution expects that URL to solr core is in certain shape <protocol>://<host>[<:port>]/[<rootPath>]/<core>. But what if somebody want to use solr core which is hidden behind proxy (for example on url http://example.com) ?
   Presented solution removes this constraint and gives more flexibility in shape of the endpoint.

2. Solution in this PR improves a performance a little bit. It is not significant, but at least something. The URL is created only once - in the constructor. Previously it was created on each request.

• What I see as an improvement is that instead of previous 4 configuration parameters there can be only 1. This can simplify integration into other systems. For example when somebody is storing configuration in database, he does not need to create 4 columns for these options.