feat(server): instance "name", store pipes in stdpath(state)

Problem:
- Unix sockets are created in random /tmp dirs.
  - /tmp is messy, unclear when OSes actually clear it.
  - The generated paths are very ugly. This adds friction to reasoning
    about which paths belong to which Nvim instances.
- No way to provide a human-friendly way to identify Nvim instances in
  logs or server addresses.

Solution:
- Store unix sockets in stdpath('state')
- Allow --listen "name" and serverstart("name") to given a name (which
  is appended to a generated path).

TODO:
- is stdpath(state) the right place?

test/functional/terminal/api_spec.lua

@@ -6,7 +6,7 @@ if helpers.pending_win32(pending) then return end
 
 describe('api', function()
   local screen
-  local socket_name = "Xtest_functional_api.sock"
+  local socket_name = "./Xtest_functional_api.sock"
 
   before_each(function()
     helpers.clear()
@@ -29,7 +29,7 @@ describe('api', function()
       {4:~                                                 }|
       {4:~                                                 }|
       {4:~                                                 }|
-      ]]..socket_name..[[                         |
+      ]]..socket_name..[[                       |
       {3:-- TERMINAL --}                                    |
     ]])