Add selectbox for server name

[ossinkine]

I've added select box for selecting server name but not just a text field.

I've implemented it for MongoDB only because PDO stores SERVER data in a text field and I have no idea how to select one of parameters from it.

Also an index is required for performance purpose.

[ossinkine]

@glensc if this is ok for you I'll try to add tests

[glensc]

Are you able to rebase against current 0.20.x, if so, add row to compatibility matrix that was added to readme in https://github.com/perftools/xhgui/pull/461

if not, I'll rebase myself and force push its branch

[ossinkine]

I'll do

[ossinkine]

I've added a test for a new searcher functionality and also added a row to the matrix you requested

[ossinkine]

@glensc Please review

[ossinkine]

@glensc when do you plan to release 0.20.1 to be able to use this feature?

[glensc]

I was hoping someone helps out with automatic releases:

• https://github.com/perftools/xhgui/issues/427