Destroy RPC handle in ecore idler

Destroy RPC handle in ecore idler

- Issue:
Invoking tts_unprepare() in event callbacks is allowed and this may
cause segmentation fault by destroying RPC handle in the callback which
is invoked by RPC message.

- Solution:
The app can call tts_unprepare() in the callback. However, this
invocation may cause segmentation fault by destroying the RPC handle in
the callback of the RPC handle itself.
To solve this issue, this patch fixes the RPC handle destruction logic.
Through this patch, the client lib will destroy the handle in ecore idler
callback. This behavior can avoid wrong handle destruction.

Change-Id: I01a2ae528be8f0fa8683c608b3786b5db282d0e1
Signed-off-by: Suyeon Hwang <stom.hwang@samsung.com>
(cherry picked from commit 96dd49ebd12b1c1853071cfe5e570e183c6133ee)