vcl: allow vcl worker index to be set by applications

vcl: allow vcl worker index to be set by applications

When using vppcom_session* apis to setup TCP sessions in applications build outside of the VPP repository, it is necessary to set the worker_index explicitly when these apis are called from the none-VCL worker threads. An example is when data is to be sent to the TCP session that is originated from a different thread, like the main program thread or from the bin api thread. This change allows the application to set it.

Type: fix

Signed-off-by: IJsbrand Wijnands <ice@cisco.com>
Change-Id: I37f3654a49ea9a8cf3a0d3d0e672583018c12299
(cherry picked from commit 6017ff0dd7a27c062d0ad4687bfc70a69747ac55)