basesrc: Make locking of the segment a bit more strict and update documentation

basesrc: Make locking of the segment a bit more strict and update documentation

Updating the segment values must only be done while holding the
STREAM_LOCK and OBJECT_LOCK. This means, reading can be done as
long as one of them is held, not both, which removes some lock-unlock
blocks from performance critical code paths.

Also document, that gst_base_src_set_format() *must* be called in
states <= READY and add an assertion for this. Changing the format
later will completely mess up the segment information.