refactor(bigtable): move ClientConfigurationManager into internal/ses… - #12

…sion

The classic Client used to construct and own the CCM, then inject it into
session.NewSessionClient so per-pool listeners could register. CCM is
fundamentally session-shaped (its consumers are the session pool sizing
listener and the classic/session traffic diverter), so let the session
tier own its construction and lifecycle.

session.NewSessionClient now dials Bigtable, builds the CCM bound to its
own stub, and starts polling. sessionClient.Close cascades: CCM.Close
(which barriers in-flight listener callbacks) before tearing down the
gRPC connection. A new OnSessionLoad method on the SessionClient
interface lets the classic Client wire the diverter without re-exposing
internal/transport types.

Drive-by fixes uncovered by the refactor:

- Diverter bootstrap was NewDiverter(1.0), which routed everything
  through a not-yet-existent session pool when EnableSessionPool=false.
  Bootstrap is now 0.0 (safe all-classic default); the OnSessionLoad
  listener is the single writer that raises it.

- CCM listeners fired on every successful poll regardless of whether the
  config had changed, so SessionPoolImpl.UpdateConfig and
  Diverter.SetSessionLoad were re-invoked on every 5-minute tick.
  AddSessionLoadListener and AddSessionPoolListener now cache the value
  they last delivered and short-circuit on unchanged polls, mirroring
  Java's ListenerEntry.maybeNotify(oldValue, newValue) equality check.

Change-Id: Ie31aba57d2f31e13378c439677898ae9a847bf23