fix(balancer) update default size from 100 to 10000

The original size was way to low to guarantee proper random
distribution beyond  3-4 targets. The recommended size is 100
slots per target. The new default covers up to 100 targets.

The approximate footprint per Upstream:
- 65535 slots, 100 targets: 1.6MB
- 10000 slots, 100 targets: 375kB

This fix should have been included in the latest update of the
DNS library dependency.

Related #3220 
From #3296

kong/dao/schemas/upstreams.lua

@@ -3,7 +3,7 @@ local utils = require "kong.tools.utils"
 local match = string.match
 local sub = string.sub
 
-local DEFAULT_SLOTS = 100
+local DEFAULT_SLOTS = 10000
 local SLOTS_MIN, SLOTS_MAX = 10, 2^16
 local SLOTS_MSG = "number of slots must be between " .. SLOTS_MIN .. " and " .. SLOTS_MAX
 

spec-old-api/01-unit/007-entities_schemas_spec.lua

@@ -719,7 +719,7 @@ describe("Entities Schemas", function()
   --
 
   describe("Upstreams", function()
-    local slots_default, slots_min, slots_max = 100, 10, 2^16
+    local slots_default, slots_min, slots_max = 10000, 10, 2^16
 
     it("should require a valid `name` and no port", function()
       local valid, errors, check

spec/01-unit/007-entities_schemas_spec.lua

@@ -793,7 +793,7 @@ describe("Entities Schemas", function()
   --
 
   describe("Upstreams", function()
-    local slots_default, slots_min, slots_max = 100, 10, 2^16
+    local slots_default, slots_min, slots_max = 10000, 10, 2^16
 
     it("should require a valid `name` and no port", function()
       local valid, errors, check

spec/02-integration/04-admin_api/07-upstreams_routes_spec.lua

@@ -3,7 +3,7 @@ local dao_helpers = require "spec.02-integration.03-dao.helpers"
 local cjson = require "cjson"
 local DAOFactory = require "kong.dao.factory"
 
-local slots_default, slots_max = 100, 2^16
+local slots_default, slots_max = 10000, 2^16
 
 local function it_content_types(title, fn)
   local test_form_encoded = fn("application/x-www-form-urlencoded")