change 'maxsockets' default value back to 15

The default value for 'maxsockets' was changed during the refactoring
in #2878 from 50 to 'Inifinity', this PR changes it to the more
accurate value of 15, which was the default used in:
https://github.com/npm/make-fetch-happen/blob/785af652ec0c8f108a43004903afd2183af93904/agent.js#L15

Fixes #2978

PR-URL: #2979
Credit: @wallrat
Close: #2979
Reviewed-by: @ruyadorno

Co-authored-by: Gar <[email protected]>

Author: 2 people

docs/content/using-npm/config.md

@@ -795,7 +795,7 @@ Show extended information in `npm ls` and `npm search`.
 
 #### `maxsockets`
 
-* Default: Infinity
+* Default: 15
 * Type: Number
 
 The maximum number of connections to use per origin (protocol/host/port

lib/utils/config/definitions.js

@@ -1154,7 +1154,7 @@ define('long', {
 })
 
 define('maxsockets', {
-  default: Infinity,
+  default: 15,
   type: Number,
   description: `
     The maximum number of connections to use per origin (protocol/host/port

tap-snapshots/test-lib-utils-config-describe-all.js-TAP.test.js

@@ -674,7 +674,7 @@ Show extended information in \`npm ls\` and \`npm search\`.
 
 #### \`maxsockets\`
 
-* Default: Infinity
+* Default: 15
 * Type: Number
 
 The maximum number of connections to use per origin (protocol/host/port