sql: Document sql::StatementID's thread safety properties.

This CL:

1) Extracts StatementID from sql/connection.{h,cc} into its own files
   sql/statement_id.{h,cc}.
2) Adds tests specifically targeting sql::StatementID.
3) Replaces passing StatementID by const reference with passing by
   value.
4) Removes the StatementID constructor that takes a name, and updates
   the call sites to use the source file+line constructor.

Bug: 863724
Change-Id: I5bc6e160fe294189d3e9a2baa8f6be61b21c38d2
Reviewed-on: https://chromium-review.googlesource.com/1137915
Commit-Queue: Victor Costan <[email protected]>
Reviewed-by: Scott Violet <[email protected]>
Reviewed-by: Chris Mumford <[email protected]>
Cr-Commit-Position: refs/heads/master@{#576295}
10 files changed