re #268, #270, updated version of leveldb from google/leveldb master

[myf]

• deps/leveldb/leveldb-1.18-ea992b467b9f3c892ae87267ba908e97d532b1b2/ is
  pulled from the master branch of https://github.com/google/leveldb at
  commit ea992b467b9f3c892ae87267ba908e97d532b1b2
• added LEVELDB_PLATFORM_UV line in port/port.h
• modified leveldb.gyp for the new version and renamed
  db/leveldb_main.cc to db/leveldbutil.cc as modified by google

[myf]

@rvagg google/leveldb v1.18 was published 2 years ago, however there are quite a few improvements since then, including the support for ARMv8 and MIPS support which dated as far back as 2014. It would be great if we upgrade leveldown when leveldb v1.19 comes out but I have no idea how far down the road this would happen.

this pr pulled the latest master of google/leveldb at commit ea992b467b9f3c892ae87267ba908e97d532b1b2, with modified port.h to include port_uv and also modified leveldb.gyp to change a file name from leveldb_main.cc to leveldbutil.cc based on upstream.

The build succeeded and all the tests passed. On my odroid c2 the build was working properly to support ARMv8.

This is an alternative to pull request #270 to support ARMv8 and MIPS. Instead the entire leveldb directory is updated to the latest master. Also since google/leveldb is it's own repository, should we do git submodules instead?

please comment :)

[martinheidegger]

1.19 has since been published. It might be good to update this PR?

[myf]

@martinheidegger you are right, since 1.19 is merged into the master this PR was intended to fix the same thing before the release of 1.19. will close after i test it on my AARCH64 system :)

[myf]

this is fixed with #299 that pulls leveldb 1.19 which added AARCH64 and MIPS as supported platforms.