[esan] Add generic resizing hashtable

Summary:
Adds a new, generic, resizing hashtable data structure for use by esan
tools.  No existing sanitizer hashtable is suitable for the use case for
most esan tools: we need non-fixed-size tables, parameterized keys and
payloads, and write access to payloads.  The new hashtable uses either
simple internal or external mutex locking and supports custom hash and
comparision operators.  The focus is on functionality, not performance, to
catalyze creation of a variety of tools.  We can optimize the more
successful tools later.

Adds tests of the data structure.

Reviewers: aizatsky

Subscribers: vitalybuka, zhaoqin, kcc, eugenis, llvm-commits, kubabrecka

Differential Revision: https://reviews.llvm.org/D22681

llvm-svn: 278024