Avoid startup/shutdown crashes [by avoiding static non-POD globals].

Avoid startup/shutdown crashes [by avoiding static non-POD globals].

Squid crashes on startup when the parent process exit()s after fork()ing
the kid process. Squid may also crash on shutdown after exiting main().

In both cases, the crashes are build- and environment-specific. Many
environments show no problems at all. Even disabling compiler
optimizations may prevent crashes. When crashes do happen, their
symptoms (e.g., backtrace) point to problems during destruction of
global objects, but the details point to innocent objects (e.g., PortCfg
or SSL_CTX).

In some environments, the following malloc error is printed on console
before the crash: "corrupted double-linked list".

This change replaces two StatHist globals used for SBuf statistics
collection with always-available singletons. The replaced globals could
be destructed before the last SBuf object using them, leading to memory
corruption (that would eventually crash Squid).

There are probably more such globals.