nimble/mesh: Fix missing SAR timeout in Proxy Server
nimble/mesh: Fix missing SAR timeout in Proxy Server

Mesh Profile 1.0 Section 6.6:
"The timeout for the SAR transfer is 20 seconds. When the timeout expires,
the Proxy Server shall disconnect."

This allows passing MESH/SR/PROX/BV-05-C.

apache/mynewt-nimble,apache/mynewt-nimble,apache/mynewt-nimble,apache/mynewt-nimble,apache/mynewt-nimble