Implement pjsua_codec_set_param()

[nanang]

Currently the API is there with empty implementation.

[nanang]

In r3074:

• Added new API pjmedia_codec_mgr_set_default_param() to set/update default codec parameter and implemented pjsua_codec_set_param() based on it.
• Added mutex in codec manager to protect states manipulations.
• Modified API pjmedia_codec_mgr_init() to add pool factory param, pool is needed to allocate mutex and codec params.
• Added new API pjmedia_codec_mgr_destroy(), e.g: to release pools.

Minor/related updates:

• Updated passthrough codec AMR to regard peer's mode-set setting.
• Fixed VAS audio device to apply AMR encoding bitrate setting.
• Fixed IPP codec codec_open() to update AMR bitrate info (for stream) when AMR encoding bitrate is not using the default, e.g: requested by peer via format param 'mode-set' in SDP.

[bennylp]

In r3075:

• Undo r3074 above as it is missing updated codec.h hence causing build errors (thanks Michael Bradley Jr for the report)

[nanang]

In r3078:

• Recommit r3074 with updated codec.h.

[nanang]

In r3080:

• Minor fixed missing pjmedia_codec_mgr_destroy() in pjmedia_endpt_destroy().

[nanang]

In r3083:

• Updated codecs docs, more detail info about codec settings.

[bennylp]

In r3100:

• fixed compile error caused by corrupted version of passthrough.c when re-commiting r3074 (in r3078).

(Note: this changeset was originally done by Nanang, recheckedin here due to moving svn repository and trac to new location)