Fix windows compatibility of memwatch.cc

[curvedriver]

Fixes windows compatibility of memwatch.cc which is broken since #3 .
I used uv_hrtime() instead of gettimeofday to get gc_ts in microseconds.

npm install of v1.0.2 and v.2.0.0 shows the following error on a windows machine:

C:\work\myproject\node_modules\@airbnb\node-memwatch\src\memwatch.cc(19): fatal error C1083: Cannot open include file: 'sys/time.h': No such file or directory [C:\work\myproject\node_modules\@airbnb\node-memwatch\build\memwatch.vcxproj]
gyp ERR! build error 
gyp ERR! stack Error: `C:\Program Files (x86)\Microsoft Visual Studio\2017\BuildTools\MSBuild\15.0\Bin\MSBuild.exe` failed with exit code: 1
gyp ERR! stack     at ChildProcess.onExit (C:\Program Files\nodejs\node_modules\npm\node_modules\node-gyp\lib\build.js:191:23)
gyp ERR! stack     at ChildProcess.emit (events.js:198:13)
gyp ERR! stack     at Process.ChildProcess._handle.onexit (internal/child_process.js:248:12)
gyp ERR! System Windows_NT 10.0.19041
gyp ERR! command "C:\\Program Files\\nodejs\\node.exe" "C:\\Program Files\\nodejs\\node_modules\\npm\\node_modules\\node-gyp\\bin\\node-gyp.js" "rebuild"
gyp ERR! cwd C:\work\myproject\node_modules\@airbnb\node-memwatch
gyp ERR! node -v v10.22.0
gyp ERR! node-gyp -v v5.1.0
gyp ERR! not ok
npm WARN optional SKIPPING OPTIONAL DEPENDENCY: [email protected] (node_modules\fsevents):
npm WARN notsup SKIPPING OPTIONAL DEPENDENCY: Unsupported platform for [email protected]: wanted {"os":"darwin","arch":"any"} (current: {"os":"win32","arch":"x64"})    

npm ERR! code ELIFECYCLE
npm ERR! errno 1
npm ERR! @airbnb/[email protected] install: `node-gyp rebuild`
npm ERR! Exit status 1
npm ERR!
npm ERR! Failed at the @airbnb/[email protected] install script.
npm ERR! This is probably not a problem with npm. There is likely additional logging output above.