gdb: change single_displaced_buffer_manager into multiple_displaced_buffer_manager

gdb: change single_displaced_buffer_manager into multiple_displaced_buffer_manager

single_displaced_buffer_manager, introduced in a previous patch, is
responsible for managing the access to a single displaced step buffer.
We'll want to be able to use more than one, so change it to
multiple_displaced_buffer_manager, which receives a collection of
buffer addresses at construction, instead of a single buffer address.

The callers keep passing a single buffer, however, so there is not
expected behavior change.