add options in core to enable default app for out-of-rdsn api invocation without explicit dsn_mimic_app calls

[imzhenyu]

No description provided.

[imzhenyu]

//
// we allow multiple apps in the same process in rDSN, and each app (service_app_spec)
// has its own rpc/thread/disk engines etc..
// when a rDSN call is made in a thread not belonging to any rDSN app,
// developers need to call dsn_mimic_app to designated which app this call and subsequent
// calls belong to.
// this is kinds of tedious sometimes, we therefore introduce enable_default_app_mimic
// option here, which automatically starts an internal app which does nothing but serves
// those external calls only. This will release the developers from writing dsn_mimic_app
// when they write certain codes, esp. client side code.
//
bool enable_default_app_mimic;

[imzhenyu]

done in #92
@qinzuoyan @glglwty please help review

[imzhenyu]

merged.