logd: switch to unordered_map from BasicHashtable

(charry pick from commit 511338dd)

BasicHashtable is relatively untested, move over to
a C++ template library that has more bake time.

Bug: 20419786
Bug: 21590652
Bug: 20500228
Change-Id: I926aaecdc8345eca75c08fdd561b0473504c5d95