This commit makes a few steps towards removing the special status
of `mainLog` as “template” for the secondary loggers.

It does so as follows:

- it lifts the global variables `showLog` into a field of `logging`,
  which is shared across all loggers. This does not change the
  behavior but makes the code slightly easier to understand.

- it makes the `stderrThreshold` parameter common to all
  loggers. Previously, the code was written to suggest that each logger
  could configure their stderrThreshold separately, however this was
  never used throughout the code base. We also have no recorded demand
  for such a fine grained configuration. This patch thus simplifies the
  code to bring the parameter into the common `logging` struct. This
  also makes the code slightly easier to understand and reason about.

- it makes the `logMaxFileSize` and `logCombinedMaxFileSize`
  parameters configurable per logger. They were previously common to all
  loggers.  Even though the code does not yet allow separate
  configurations for separate loggers, this change provisionally makes
  sense because we already know there is demand for keeping audit
  logs forever and debug logs only for a short time.

- it makes the handling of the `syncWrites` flag simpler. Previously,
  this parameter existed separately for each logger. It could also be
  toggled on and off across all loggers simultaneously, by
  `SetSync()`. However, the only caller of `SetSync()` ever set it to
  `true`. This patch thus simplifies the handling by making the
  per-logger `syncWrite` flag immutable (so it can be lifted out of the
  `mu` sub-struct), and have the file write log look into a (new)
  `logging.syncWrites`, which can be flipped by `SetSync()`.
  This also makes the code simpler overall.

- as a result of the above changes, the constructor
  `NewSecondaryLogger()` now needs only peek at `logging` and does not
  need `mainLog` any more.

- similarly, the `Scope()` facilities in `test_log_scope.go` can
  be simplified since they only affect settings across all loggers.

Release note: None

This change extracts the protobuf definitions into a separate package
`logpb`. This disentangles the `log` package from `logpb` and enables
other packages to depend on `logpb` without bringing the entire `log`
infrastructure. It also clarifies the notion that the structs defined
in `logpb` have a stable marshalled representation.

Additionally, this change aliases the severity constants into
shorthands in a new sub-package `severity`, which somewhat simplifies
usage in other packages.

Finally, this change introduces a code generator for the more
repetitive interfaces.

Release note: None