Lookup session by capability

[tomv564]

Fixes #562 where view and command-based LSP code only looks at scope to find the most suitable session.
In a case where two servers run for the same scope (eg. typescript and eslint server on source.js), LSP would pick the first session, instead of checking which server provides the relevant capability.

Implementation details:

• Forced all lookups to consider multiple servers by changing scope_config -> scope_configs
• session_for_view and LspTextCommand.client_with_capability take capability parameter.
• Use iterators to save some effort of calculating 2nd, 3rd matching session if only the first session is needed.
• Attempt to fix some timings on completion tests on travis.

[coveralls]

Coverage increased (+0.1%) to 37.064% when pulling e7660f8 on lookup-by-capability into 7bb8de0 on master.