Make chr() work with unicode

* Update docs to reference chr() instead of char()
* Make chr() work with unicode code points instead of ascii code points to match the documentation
Skip call to get partitions if domain is NONE
Fix incorrect parsing of function arguments

The current parser incorrectly allows function calls with the following syntax:

    foo(,1,2)
    foo(DISTINCT ,x)
    foo(DISTINCT)
Remove unused parser rule
Fix race condition in HttpRemoteTask

There is a race condition in HttpRemoteTask where it can race on setting the value of the needsUpdate flag. The potential result of this is that we can silently drop a failed HTTP request and not notify any of the callers.
Add AsyncSemaphore
Throttle Presto to a maximum number of concurrent queries
Bound the number of queued queries in Presto
Fix JDBC client hanging the JVM on shutdown

The IO pool was not specified for the HTTP client which caused the threads
to be created as non-daemon threads (which prevented the JVM from exiting).

Conflicts:
	presto-docs/src/main/sphinx/release/release-0.76.rst