Used shared_ptr to keep reference to a TaskExecutor

One motivation for this is to make it compatible with SemiFuture::thenRunOn

Here's a list of classes that keep a raw pointer reference to TaskExecutor:

Fetcher
RemoteCommandRetryScheduler
ReplicaSetMonitor

Under src/db/repl:
AbstractAsyncComponent
CollectionCloner
DataReplicatorExternalStateImpl
DatabaseCloner
InitialSyncer
MultiApplier
OplogApplier
Reporter
ScatterGatherRunner
SyncSourceResolver