Make eldoc faster (and slightly smarter)

[galaunay]

PR Summary

Make eldoc display faster by avoiding multiple calls to the RPC.
Also enhance a bit the kind of information eldoc displays:

• Avoid displaying infos about basic builtins
• In case we can display a calltip and a function at the same time, favour the function if the doc is available.

PR checklist

Please make sure that the following things have been addressed (and check the relevant checkboxes):

• Commits respect our guidelines
• Tests are passing properly (see here on how to run Elpy's tests)

[coveralls]

Coverage decreased (-0.2%) to 92.145% when pulling 9440115 on galaunay:faster-eldoc into d02f4da on jorgenschaefer:master.