basesrc: getters for pool and allocator

basesrc: getters for pool and allocator

Sometimes the sources would use the buffer pool or the memory allocator for
something else than just allocating output buffers; for example, querying for
different parameters, such as a bigger number of buffers to allocate by the
pool.

This patch expose a two getters accessors: one for the buffer pool and the
other for the memory allocator.