Related: #4470#4473
Motivation:
A user might want to:
- implement dynamic mapping from hostname to SslContext
- server large number of domain names whose SslContext can be
initialized lazily and destroyed when unused
Modifications:
- Let SniHandler accept Mapping<String, SslContext> as well as
DomainNameMapping
- Make the default constructor of SslContext so that a user can create
his or her own SslContext wrapper
Result:
Flexibility