Minor cleanup in PagingSelfAwareStructuredLogger

[morgen-peschke]

Materialize the paged message before interacting with it.

Previously the message would be materialized multiple times:

• 1 : in call to addMsgCtx
• 3 or 1 + 3p : in call to pagedLogging
  • Lower Bound (single page): 3
    • 1 : calculate the number of pages
    • 1 : in call to addPageCtx (calculate the page length)
    • 1 : in call to logOpWithCtx
  • Upper Bound (p pages): 1 + 3p
    • 1 : calculate the number of pages
    • 1p : msg is materialized every time a page is sliced from the message
    • 1p : in call to addPageCtx (calculate the page length)
    • 1p : in call to logOpWithCtx

Materializing early gives these improvements:

• Single page: from 4 times to 1 time
• Multiple pages: from 2 + 3p times to 1 time

Removes some by-name parameters that were always called with materialized values.

Simplifies unneeded calls to Show on Int or String values, which doesn't have any particular benefit, particularly if it's inside of a string interpolation anyway.