liblog: have writers handle their own state

Remove the transport available and open functions since the writers
are able to manage their own state.  Remove the initialization dance
with write_to_log, since it is unneeded once this is removed as well.

Remove the global lock around the close() functions as correct locking
has been added to the writers in a previous change.

Test: logging works, liblog-unit-tests
Change-Id: If7fa11e773763d0b5fcb2e696ad1c88ff4a4cfdf